IN vs. EXISTS ...... NOT IN vs. NOT EXISTS

I have statements where i use the above oracle functions. IN and EXISTS seem _not_ to be interchangable (same for negation) Could some explain the differnece in behavior between those functions (beside performace.. IN is slower I think)?. I find it tricky to use one vs. the other. I end up having to experimenting with permutation
to get the logic right.
thanks
Kal.

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