Miss chance trumps a natural 20, for reasons stated above. Imagine that an effect gave my attackers a 100% miss chance, and someone who didn't know that attacked me and rolled a 20. He still misses, because there's a 100% miss chance. The miss chance sometimes represents attacking the wrong place, as in invisiblity as mentioned above, and it sometimes means that the defender wasn't there to be hit at that moment (a la Blink). That said, I like to roll the attack roll first or at that same time, for reasons mentioned by shilsen. Then again, when players threaten a critical hit against a construct or undead and begins to roll the confirm, I don't stop them until afterwards for the same reason ^^ As far as natural 20s go, I've personally never liked the natural 20 always succeeds and natural 1 always fails rule, so I use the variant rule in the DMG wherein a 20 means roll again and add both rolls to your bonus (and keep rolling again and again if you keep getting 20). This helps deal with the fact that Sheri the peasant girl who has never battled in her life (attack roll total -2) can hit Baxtrius, Lord of Battles (AC 52) 5% of the time, even if he has 90% cover, etc. It also means that the bad guy can't hire 30 weak apprentices and expect the high-level heroes to fail on their strong save against at least 1. Overall, this variant favours the PCs because it means that hordes of worthless weenies might not threaten them any more.