A periodic action is a viewmodel action that runs automatically over and over again until the Disabled expression is True.

In the viewmodel editor;

You typically use this for;

• Refreshing a view in WPF with selfVM.Refresh if for example your root object is in a state that changes server side.
• Automatically navigate on a change in the database (for example after login when a current user has ben set)
• Updating a calculated value or transient collection/association that you show on screen.

See also MDrivenServer periodic server-side actions which is a different thing, but usually works in conjuction with viewmodel periodic actions.