From the very first episode, we've known there is a history between Walter Bishop and the people in charge of Massive Dynamics. William Bell and Walter were research partners as well as friends, while there are clearly some negative feelings between Nina Sharp and Walter. One thing's for certain: these three people have a long and mixed past.
As the series goes on, we learn that these people have shared a history of lab fires, experiments that are ethically questionable, and so much more. The story of what came before Fringe is just as entertaining as the show itself, and this fact is perfectly illustrated by the popularity of flashback episodes like "Peter" in season two. This rich backstory also led to the production of a comic book prequel that was written by show writer Zach Whedon, brother of Joss Whedon.
