Yahtzee

I know that the probability of any one die hitting one number (assuming 'perfect dice') is 1/6. But how do you determine the odds of hitting a certain 'hand' in a game such as Yahtzee? The part that baffles me is the fact that there is a maxium of 15 rolls, a miniumum of 5, you may hit the Yahtzee on any of the rolls, you may not hit it at all, you may hit 3 the first time and toss 2 the second time, etc. Could someone help me out or direct me to a reference? Thank you.