This book offers a comprehensive introduction to learning classifier systems (LCS) -- or more generally, rule-based evolutionary online learning systems. LCSs learn interactively -- much like a neural network -- but with an increased adaptivity and flexibility. This book provides the necessary background knowledge on problem types, genetic algorithms, and reinforcement learning as well as a principled, modular analysis approach to understand, analyze, and design LCSs. The analysis is exemplarily carried through on the XCS classifier system -- the currently most prominent system in LCS research. Several enhancements are introduced to XCS and evaluated. An application suite is provided including classification, reinforcement learning and data-mining problems. Reconsidering John Holland's original vision, the book finally discusses the current potentials of LCSs for successful applications in cognitive science and related areas.