This volume is dedicated to Heinz Post who proposed a rational model of scientific discovery. His account draws attention to the formal flaws in theories that motivate theory modification, the correspondence relations that hold between old and new theories and the cross-theoretic retention of symmetry and conservation principles. Exploring Post's model from a variety of perspectives, the contributors draw on a wide range of case studies from physics, chemistry and biology. This is the first work to examine one such model of heuristics in the context of detailed examples from science itself. It will be of interest to teachers, researchers and graduate students in both the history and philosophy of science and can be used as a textbook in advanced courses on scientific method.