diff --git a/Indented.ScriptAnalyzerRules/Indented.ScriptAnalyzerRules.psd1 b/Indented.ScriptAnalyzerRules/Indented.ScriptAnalyzerRules.psd1
index 58a317b..0d47a17 100644
Binary files a/Indented.ScriptAnalyzerRules/Indented.ScriptAnalyzerRules.psd1 and b/Indented.ScriptAnalyzerRules/Indented.ScriptAnalyzerRules.psd1 differ
diff --git a/Indented.ScriptAnalyzerRules/public/helper/Resolve-ParameterSet.ps1 b/Indented.ScriptAnalyzerRules/public/helper/Resolve-ParameterSet.ps1
index a913065..128410a 100644
--- a/Indented.ScriptAnalyzerRules/public/helper/Resolve-ParameterSet.ps1
+++ b/Indented.ScriptAnalyzerRules/public/helper/Resolve-ParameterSet.ps1
@@ -93,9 +93,11 @@ function Resolve-ParameterSet {
 
                     $errorRecord = [ErrorRecord]::new(
                         [InvalidOperationException]::new(
-                            ('{0}: Ambiguous parameter set: {1}' -f
-                                $CommandInfo.Name,
-                                ($candidateSets.Name -join ', ')
+                            (
+                                '{0}: Ambiguous parameter set: {1}' -f @(
+                                    $CommandInfo.Name
+                                    $candidateSets.Name -join ', '
+                                )
                             )
                         ),
                         'AmbiguousParameterSet',
diff --git a/Indented.ScriptAnalyzerRules/public/rules/AvoidOutOfScopeVariables.ps1 b/Indented.ScriptAnalyzerRules/public/rules/AvoidOutOfScopeVariables.ps1
new file mode 100644
index 0000000..b550908
--- /dev/null
+++ b/Indented.ScriptAnalyzerRules/public/rules/AvoidOutOfScopeVariables.ps1
@@ -0,0 +1,151 @@
+using namespace Microsoft.Windows.PowerShell.ScriptAnalyzer.Generic
+using namespace System.Management.Automation.Language
+using namespace System.Collections.Generic
+
+function AvoidOutOfScopeVariables {
+    <#
+    .SYNOPSIS
+        AvoidOutOfScopeVariables
+
+    .DESCRIPTION
+        Functions should not use out of scope variables unless a scope modifier is explicitly used to indicate source scope.
+    #>
+
+    [CmdletBinding()]
+    [OutputType([Microsoft.Windows.PowerShell.ScriptAnalyzer.Generic.DiagnosticRecord])]
+    param (
+        [FunctionDefinitionAst]$ast
+    )
+
+    # Special variables:
+    # [PowerShell].Assembly.GetType('System.Management.Automation.SpecialVariables').GetFields('Static,NonPublic') |
+    #    Where-Object FieldType -eq ([string]) |
+    #    ForEach-Object GetValue($null)
+    $specialVariables = [HashSet[string]]::new([IEqualityComparer[string]][StringComparer]::OrginalIgnoreCase)
+    $whitelist = @(
+        '_'
+        '?'
+        '^'
+        '$'
+        'args'
+        'ConfirmPreference'
+        'CurrentlyExecutingCommand'
+        'DebugPreference'
+        'EnabledExperimentalFeatures'
+        'env:PATHEXT'
+        'error'
+        'ErrorActionPreference'
+        'ErrorView'
+        'ExecutionContext'
+        'false'
+        'foreach'
+        'HOME'
+        'Host'
+        'InformationPreference'
+        'input'
+        'IsCoreCLR'
+        'IsLinux'
+        'IsMacOS'
+        'IsWindows'
+        'LASTEXITCODE'
+        'LogCommandHealthEvent'
+        'LogCommandLifecycleEvent'
+        'LogEngineHealthEvent'
+        'LogEngineLifecycleEvent'
+        'LogProviderHealthEvent'
+        'LogProviderLifecycleEvent'
+        'LogSettingsEvent'
+        'Matches'
+        'MaximumHistoryCount'
+        'MyInvocation'
+        'NestedPromptLevel'
+        'null'
+        'OFS'
+        'OutputEncoding'
+        'PID'
+        'ProgressPreference'
+        'PSBoundParameters'
+        'PSCmdlet'
+        'PSCommandPath'
+        'PSCulture'
+        'PSDebugContext'
+        'PSDefaultParameterValues'
+        'PSEdition'
+        'PSEmailServer'
+        'PSHOME'
+        'PSItem'
+        'PSLogUserData'
+        'PSModuleAutoLoadingPreference'
+        'PSNativeCommandArgumentPassing'
+        'PSScriptRoot'
+        'PSSenderInfo'
+        'PSSessionApplicationName'
+        'PSSessionConfigurationName'
+        'PSStyle'
+        'PSUICulture'
+        'PSVersionTable'
+        'PWD'
+        'ShellId'
+        'StackTrace'
+        'switch'
+        'this'
+        'true'
+        'VerboseHelpErrors'
+        'VerbosePreference'
+        'WarningPreference'
+        'WhatIfPreference'
+    )
+    foreach ($name in $whitelist) {
+        $null = $specialVariables.Add($name)
+    }
+
+    $declaredVariables = @{}
+    $ast.Body.FindAll(
+        {
+            param (
+                $ast
+            )
+
+            $ast -is [AssignmentStatementAst] -and
+            $ast.Left.VariablePath.IsUnqualified
+        },
+        $true
+    ) | ForEach-Object {
+        $userPath = $_.Left.VariablePath.UserPath
+        if ($declaredVariables.Contains($userPath)) {
+            if ($_.Extent.StartOffset -lt $declaredVariables[$userPath]) {
+                $declaredVariables[$userPath] = $_.Extent.StartOffset
+            }
+        } else {
+            $declaredVariables[$userPath] = $_.Extent.StartOffset
+        }
+    }
+
+    $ast.Body.FindAll(
+        {
+            param (
+                $ast
+            )
+
+            $ast -is [VariableExpressionAst] -and
+            $ast.VariablePath.IsUnqualified -and
+            $ast.Parent -isnot [ForEachStatementAst] -and
+            -not $specialVariables.Contains($ast.VariablePath.UserPath) -and
+            (
+                (
+                    $declaredVariables.Contains($ast.VariablePath.UserPath) -and
+                    $ast.Extent.StartOffset -lt $declaredVariables[$ast.VariablePath.UserPath]
+                ) -or
+                -not $declaredVariables.Contains($ast.VariablePath.UserPath)
+            )
+        },
+        $true
+    ) | ForEach-Object {
+        [DiagnosticRecord]@{
+            Message  = 'The function {0} contains an out of scope variable: ${1}' -f $ast.Name, $_.VariablePath.UserPath
+            Extent   = $_.Extent
+            RuleName = $myinvocation.MyCommand.Name
+            Severity = 'Warning'
+        }
+    }
+}
diff --git a/Indented.ScriptAnalyzerRules/tests/unit/rules/AvoidOutOfScopeVariables.tests.ps1 b/Indented.ScriptAnalyzerRules/tests/unit/rules/AvoidOutOfScopeVariables.tests.ps1
new file mode 100644
index 0000000..bbc5ea1
--- /dev/null
+++ b/Indented.ScriptAnalyzerRules/tests/unit/rules/AvoidOutOfScopeVariables.tests.ps1
@@ -0,0 +1,68 @@
+Describe AvoidOutOfScopeVariables {
+    BeforeAll {
+        $ruleName = @{ RuleName = 'AvoidOutOfScopeVariables' }
+    }
+
+    It 'Triggers when <Why> is used in "<Script>"' -TestCases @(
+        @{
+            Why    = 'a script uses a variable from a parent scope'
+            Script = {
+                function testFunction {
+                    if ($variable) { }
+                }
+            }
+        }
+        @{
+            Why    = 'a variable is used before it is declared'
+            Script = {
+                function testFunction {
+                    if ($variable) { }
+                    $variable = $true
+                }
+            }
+        }
+    ) {
+        Invoke-CustomScriptAnalyzerRule -ScriptBlock $script @ruleName | Should -Not -BeNullOrEmpty
+    }
+
+    It 'Does not trigger when <Why> is used in "<Script>"' -TestCases @(
+        @{
+            Why    = 'the variable is assigned prior to use'
+            Script = {
+                function testFunction {
+                    $variable = $true
+                    if ($variable) { }
+                }
+            }
+        }
+        @{
+            Why    = 'a built-in variable'
+            Script = {
+                function testFunction {
+                    if ($PSVersionTable.PSVersion) { }
+                }
+            }
+        }
+        @{
+            Why    = 'an explicit variable scope'
+            Script = {
+                function testFunction {
+                    if ($Script:variable) { }
+                }
+            }
+        }
+        @{
+            Why    = 'a loop variable is used'
+            Script = {
+                function testFunction {
+                    $array = 1..5
+                    foreach ($value in $array) {
+
+                    }
+                }
+            }
+        }
+    ) {
+        Invoke-CustomScriptAnalyzerRule -ScriptBlock $script @ruleName | Should -BeNullOrEmpty
+    }
+}
diff --git a/Indented.ScriptAnalyzerRules/tests/unit/rules/AvoidThrowOutsideOfTry.tests.ps1 b/Indented.ScriptAnalyzerRules/tests/unit/rules/AvoidThrowOutsideOfTry.tests.ps1
index 2e7c079..9bb234b 100644
--- a/Indented.ScriptAnalyzerRules/tests/unit/rules/AvoidThrowOutsideOfTry.tests.ps1
+++ b/Indented.ScriptAnalyzerRules/tests/unit/rules/AvoidThrowOutsideOfTry.tests.ps1
@@ -6,7 +6,7 @@ Describe AvoidThrowOutsideOfTry {
     It 'Triggers when throw is used outside of a try block in "<Script>"' -TestCases @(
         @{
             Script = {
-                function name {
+                function testFunction {
                     [CmdletBinding()]
                     param ( )
 
@@ -16,7 +16,7 @@ Describe AvoidThrowOutsideOfTry {
         }
         @{
             Script = {
-                function name {
+                function testFunction {
                     [CmdletBinding()]
                     param ( )
 
@@ -31,7 +31,7 @@ Describe AvoidThrowOutsideOfTry {
         }
         @{
             Script = {
-                function name {
+                function testFunction {
                     [CmdletBinding()]
                     param ( )
 
@@ -52,7 +52,7 @@ Describe AvoidThrowOutsideOfTry {
     It 'Does not trigger when used inside of a try block in "<Script>"' -TestCases @(
         @{
             Script = {
-                function name {
+                function testFunction {
                     [CmdletBinding()]
                     param ( )
 
@@ -66,7 +66,7 @@ Describe AvoidThrowOutsideOfTry {
         }
         @{
             Script = {
-                function name {
+                function testFunction {
                     [CmdletBinding()]
                     param ( )
 
@@ -82,7 +82,7 @@ Describe AvoidThrowOutsideOfTry {
         }
         @{
             Script = {
-                function name {
+                function testFunction {
                     [CmdletBinding()]
                     param ( )
 
@@ -98,7 +98,7 @@ Describe AvoidThrowOutsideOfTry {
         }
         @{
             Script = {
-                function name {
+                function testFunction {
                     [CmdletBinding()]
                     param ( )
 
diff --git a/Indented.ScriptAnalyzerRules/tests/unit/rules/UseSyntacticallyCorrectExamples.tests.ps1 b/Indented.ScriptAnalyzerRules/tests/unit/rules/UseSyntacticallyCorrectExamples.tests.ps1
index 3e12714..d2cbfe1 100644
--- a/Indented.ScriptAnalyzerRules/tests/unit/rules/UseSyntacticallyCorrectExamples.tests.ps1
+++ b/Indented.ScriptAnalyzerRules/tests/unit/rules/UseSyntacticallyCorrectExamples.tests.ps1
@@ -6,21 +6,22 @@ Describe UseSyntacticallyCorrectExamples {
     Context 'Present' {
         It 'Example present, invalid parameter, returns record' {
             $script = {
-                function name {
+                function testFunction {
                     <#
                     .SYNOPSIS
-                        name
+                        testFunction
                     .DESCRIPTION
-                        name
+                        testFunction
                     .EXAMPLE
-                        name -param2
+                        testFunction -param2
 
-                        An example of name.
+                        An example of testFunction.
                     #>
                     [CmdletBinding()]
                     param (
                         $param1
                     )
+                    $param1
                 }
             }
 
@@ -32,16 +33,16 @@ Describe UseSyntacticallyCorrectExamples {
 
         It 'Example present, valid parameter, invalid parameter set, returns record' {
             $script = {
-                function name {
+                function testFunction {
                     <#
                     .SYNOPSIS
-                        name
+                        testFunction
                     .DESCRIPTION
-                        name
+                        testFunction
                     .EXAMPLE
-                        name -param1 value -param2 value
+                        testFunction -param1 value -param2 value
 
-                        An example of name.
+                        An example of testFunction.
                     #>
                     [CmdletBinding()]
                     param (
@@ -51,6 +52,8 @@ Describe UseSyntacticallyCorrectExamples {
                         [Parameter(ParameterSetName = 'two')]
                         $param2
                     )
+                    $param1
+                    $param2
                 }
             }
 
@@ -62,20 +65,21 @@ Describe UseSyntacticallyCorrectExamples {
 
         It 'Example present, not advanced function, returns null' {
             $script = {
-                function name {
+                function testFunction {
                     <#
                     .SYNOPSIS
-                        name
+                        testFunction
                     .DESCRIPTION
-                        name
+                        testFunction
                     .EXAMPLE
-                        name -param1 value -param2 value
+                        testFunction -param1 value -param2 value
 
-                        An example of name.
+                        An example of testFunction.
                     #>
                     param (
                         $param1
                     )
+                    $param1
                 }
             }
 
@@ -84,21 +88,22 @@ Describe UseSyntacticallyCorrectExamples {
 
         It 'Example present, valid parameters, no parameter set, returns null' {
             $script = {
-                function name {
+                function testFunction {
                     <#
                     .SYNOPSIS
-                        name
+                        testFunction
                     .DESCRIPTION
-                        name
+                        testFunction
                     .EXAMPLE
-                        name -param1 value
+                        testFunction -param1 value
 
-                        An example of name.
+                        An example of testFunction.
                     #>
                     [CmdletBinding()]
                     param (
                         $param1
                     )
+                    $param1
                 }
             }
 
@@ -107,16 +112,16 @@ Describe UseSyntacticallyCorrectExamples {
 
         It 'Example present, valid parameters, valid parameter set, returns null' {
             $script = {
-                function name {
+                function testFunction {
                     <#
                     .SYNOPSIS
-                        name
+                        testFunction
                     .DESCRIPTION
-                        name
+                        testFunction
                     .EXAMPLE
-                        name -param1 value
+                        testFunction -param1 value
 
-                        An example of name.
+                        An example of testFunction.
                     #>
                     [CmdletBinding()]
                     param (
@@ -126,6 +131,8 @@ Describe UseSyntacticallyCorrectExamples {
                         [Parameter(ParameterSetName = 'two')]
                         $param2
                     )
+                    $param1
+                    $param2
                 }
             }
 
@@ -134,21 +141,22 @@ Describe UseSyntacticallyCorrectExamples {
 
         It 'Example present, valid parameters, valid parameter set, repeated "command" name, returns null' {
             $script = {
-                function name {
+                function testFunction {
                     <#
                     .SYNOPSIS
-                        name
+                        testFunction
                     .DESCRIPTION
-                        name
+                        testFunction
                     .EXAMPLE
-                        name -param1 value
+                        testFunction -param1 value
 
-                        name is used to do stuff.
+                        testFunction is used to do stuff.
                     #>
                     [CmdletBinding()]
                     param (
                         $param1
                     )
+                    $param1
                 }
             }
 
@@ -159,11 +167,12 @@ Describe UseSyntacticallyCorrectExamples {
     Context 'Absent' {
         It 'Write-Error absent, returns null' {
             $script = {
-                function name {
+                function testFunction {
                     [CmdletBinding()]
                     param (
                         $param1
                     )
+                    $param1
                 }
             }