In a round about way, just about everything with the exception of geography. As an engineer, Math and English skills are key, but speech skills are important for working with other individuals. Economics gives me a point of view when designing certain mechanisms ( like points / awards ). Humanities ( ethnic studies, family studies, art, etc ) allow me to better understand and relate to other people, which is important when working in a culturally and ethnically diverse environment. All of these subjects inform my thought process as I set out to solve various problems. With that said, however, I'm currently in a place where I think to myself "Man, I'd love to use X, and wish I had studied it harder in school." Higher levels of Math, specifically. At the time ( high school ) I just didn't see the value of any class of Math beyond Algebra. Granted, at the time, teachers couldn't or wouldn't really answer the great teenage question "When am I ever going to use this stuff," and I might have made better decisions if I had, but the responsibility is ultimately mine. I'm back in school now to pick up the skills I'm lacking, but recommend doing it right the first time.