folding a hand preflop after it is raised

Just wondering what the general thought is on this scenario.

You are in Middle Position with a hand like A [img]/images/graemlins/spade.gif[/img] 6 [img]/images/graemlins/heart.gif[/img] or a low pp say 5 [img]/images/graemlins/diamond.gif[/img] 5 [img]/images/graemlins/club.gif[/img] or suited connectors like 8 [img]/images/graemlins/diamond.gif[/img] 9 [img]/images/graemlins/diamond.gif[/img] etc. you decide to call to see the flop and the pot is raised behind you.
What do we do
Call?
Fold?
scenarios could include pots with a number of callers or pots with no callers.

Thoughts?