Can you parry boss attacks?

I just started playing Dark Souls, and I was able to die to a hollow trying to parry its attacks. Eventually, I kind of got the timing down and slew him with a satisfying riposte.

Then I fought a big demon with a club bigger than I am. Does parrying work on all attacks, like this boss's giant club? Or does it only work on some attacks? How can I tell which attacks are parry-able?

Solution 1:

The general rule of thumb is that if the enemy is humanoid, and about the same size as you, you can parry it. You can't parry bosses (except for the final boss), non humanoid critters, or the giants in Anor Londo. The largest enemies you can parry are the Black Knights who are about half again as tall as you.

After you play for a while you'll get the hang of which things you can and can't critical.