Set Parameter Description in Custom Tool

[MikeMacRae]

Does anyone know how to set the description for a parameter in a custom tool? In the example below of the "Calculate Field" tool, when you click in each parameter you get a description about the input/value/item to be entered in on the right hand side. I want to add a description in a customized tool, but there doesn't seem to be an option to do so?

[DaleHoneycutt]

If you're on 10.0, right-click your tool in the catalog window and click "Item Description".  Then hit the Edit button to edit the description.

See A quick tour of documenting tools for more info

For 9.3, see An overview of documenting tools and toolboxes