G.20.1- If both players lose the game at the same time or during the resolution of a single part/event of a spell or ability, the game ends in a draw. It does not matter if one player has a more negative life total. [CompRules 2001/07/23 - 102.4]
G.20.2- Cards in the game may add additional ways for a player to lose the game. [D'Angelo 1998/02/03]
G.20.3- A player may concede the game at any time. Nothing can be done to respond to this. The game is simply over and they lost. [CompRules 2001/07/23 - 102.7]
G.20.4- A player loses if their life total is zero or less. This is a state-based effect. See Rule G.19.3. [CompRules 2001/07/23 - 102.1]
G.20.5- A player loses if they are required to draw more cards than they have left in their library. See Rule Z.6.8. This loss happens as a state-based effect just following the resolution of the spell or ability requiring the draw. [CompRules 2001/07/23 - 102.2]
G.20.6- If a player would both win and lose at the same time, they lose. [CompRules 2001/07/23 - 102.5]
G.20.7- A player loses the game as a state-based effect (see Rule T.11) if they have 10 or more Poison counters. See Rule E.10. [CompRules 2001/07/23 - 102.8]