Antibiotic resistance in pathogenic bacteria is a simple and elegant example of adaptation by natural selection. In my talk, I will show how simple experiments can be used to investigate the fundamental evolutionary processes that drive the spread and maintenance of antibiotic resistance in Pseudomonas. Antibiotic resistance comes at a fitness cost, expressed in terms of reduced competitive ability and virulence, and I will focus on addressing two issues. First, why does resistance carry a cost? Second, how do antibiotic use and natural selection allow resistance to persist over the long term in spite of its initial cost?