Wedge if possible
White is attachment. The proverb says "Respond to attachment with hane". Black can consider responding at a and b.
If the wedge (a) works, it will be more severe.
White at c is common and is discussed here.
White outside atari
A common continuation following the wedge of (see four die but six live). For White to choose this line of play, it is desireable, but not an absolute requirement, that there be a White stone in the upper left.
Jared: I think is the only move, but maybe
could be at a.
White inside atari
Another possible continuation, often seen when the two marked Black stones are part of a moyo.
Bill: Isn't there a better page title? Wedge if possible is not good general go advice. It is specific to this kind of invasion an response. Perhaps the thing to do is to make this a subpage?