How do I make an object only visible inside another?

Hey, I've been playing with the Blend Modes for this, but is not perfect.

I would like to have object B appearing only inside object A, and the parts that are not inside object A stays invisible. Any suggestion?

Appreciated! : )

Put A and B on the same layer, B above A.

Set blend mode for B to "Source in"

Oh yeah, that works dop2000, thank you! : )

I'd like to do the same but with layers. layers do as well have blending modes. however if I set layer-b to source in while having a background underneath layer-a the entire layer-b keeps being visible (and background is hidden). Is there a way to prevent this?