How do you comment out code in PowerShell?

In PowerShell V1 there's only # to make the text after it a comment.

# This is a comment in PowerShell

In PowerShell V2 <# #> can be used for block comments and more specifically for help comments.

#REQUIRES -Version 2.0

<#
.SYNOPSIS
    A brief description of the function or script. This keyword can be used
    only once in each topic.
.DESCRIPTION
    A detailed description of the function or script. This keyword can be
    used only once in each topic.
.NOTES
    File Name      : xxxx.ps1
    Author         : J.P. Blanc ([email protected])
    Prerequisite   : PowerShell V2 over Vista and upper.
    Copyright 2011 - Jean Paul Blanc/Silogix
.LINK
    Script posted over:
    http://silogix.fr
.EXAMPLE
    Example 1
.EXAMPLE
    Example 2
#>
Function blabla
{}

For more explanation about .SYNOPSIS and .* see about_Comment_Based_Help.

Remark: These function comments are used by the Get-Help CmdLet and can be put before the keyword Function, or inside the {} before or after the code itself.

You use the hash mark like this:

# This is a comment in PowerShell

Wikipedia has a good page for keeping track of how to do comments in several popular languages:

Comments

Single line comments start with a hash symbol, everything to the right of the # will be ignored:

# Comment Here

In PowerShell 2.0 and above multi-line block comments can be used:

<# 
  Multi 
  Line 
#> 

You could use block comments to embed comment text within a command:

Get-Content -Path <# configuration file #> C:\config.ini

Note: Because PowerShell supports Tab Completion you need to be careful about copying and pasting Space + TAB before comments.

It's the #.

See PowerShell - Special Characters And Tokens for special characters.