Late last year, Elon Musk gave a talk at the 67th International Astronautical Congress outlining why he believes humans need to be a multi-planetary species, and how he plans to do it. Musk's talk, which you can watch here, has also been transcribed into this helpful document. This is a plain-language description of putting humans on Mars written by one of the leaders in the game. It's worth a read (or a watch).