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Urban design is not a profession or a pursuit, that in my opinion, can be defined in any one simple way. If you are looking for a more concrete answer, my UD prof always used to say that Urban Design is What Urban Designers do. It just can't be simplified to say that it is a little larger scale than Architecture and a little smaller than planning, it is more than that. Designeres find themselves working on one small site one week, and on a streetscape the next. I have worked on busstops and lots of other stuff, too. The question is really loaded, it is maybe a bit too romantic for me to say, but exactly what an Urban Designer is, you almost have to define it yourself. It is about formulation of a community, an aesthetic, a function, and a place. As far as a concrete job, you'd probably end up as a psuedo planner in a consulting firm or an architecture firm. Read books like "Common Place" and "Downtown Urban Design" they are helpful.