Are dialogs sheets?

No.

And is it possible to have a dialog within a dialog?

No.

:-)

Sorry to be terse, but without knowing what you’re trying to do, I can’t really elaborate. This topic may interest you, though: http://www.cocoabuilder.com/archive/message/cocoa/2005/3/30/131753

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ok. What are dialogs then? :-)

There are alert panels (check out NSAlert documentation) that float above like a windows dialogue box and sheets (check out NSApplication’s documentation, the beginSheet:modalForWindow:modalDelegate:didEndSelector:contextInfo: method) which are panels that pop out of the top of whatever window they’re attached to.