I'm currently enrolled in a PhD program in control systems engineering. I'm wondering what tends tend to be the most useful subsets of this field, and consequently the most important to be well-versed in. I've read Kalman filters are very important in real world applications. Optimal Control and Convex optimization also seem to be important. My goal is ultimately to work in industry, possibly including starting my own company. I'm debating actually, if a job like this would require a PhD or just a masters. Especially if, in true startup fashion, I would have to be able to do everything including the applied digital/analog stuff and spend time learning more applied things (i'm not well versed in digital circuits at all! and this would take considerable time/effort). P.S. *I change my mind about whether I want to get a PhD or not every day even thought I'm very passionate about nonlinear dynamics, control systems, and these sorts of things. It's a complete crap-shoot, despite my interest in the field. The PhD degree, at this point (I've just started) seems to be like a master's plus teaching experience. I have no interest in teaching/becoming a professor. Research would be fine/ good depending on what I get to work on, but I am tired of the TAing stuff and just want to learn this stuff for myself. Especially when I feel mathematically literate/mature enough to just buy things like a Dover optimal control book, or a Dover adaptive control book which are like ~$9, and just learn it by reading and teaching myself.