Well, if you know he has a straight, generally a fold would be in order , unless you have crazy number of outs and you're getting the odds.
In terms of when you have the straight, it all depends on A) position b) opponent. Hard to be more specific without a specific situation. 90% of what you do in Poker depends on A) and B). It could be anywhere between a fold (unless you had the nuts obv) to an all in depending on those two.