The study of composition operators forges links between fundamental properties of linear operators and beautiful results from the classical theory of analytic functions. This book provides a self-contained introduction to both the subject and its function-theoretic underpinnings, and features a development accessible to anyone who has studied basic graduate level real and complex analysis. The work traces how such operator-theoretic issues as boundedness, compactness, and cyclicity, when studied in the context of composition operators, evolve into questions about subordination, value-distribution, angular derivatives, iteration, and functional equations; and it carefully develops each of these classical topics.