This book is a concise guide to the theory and design of phase-locked loop circuits. It is written from an engineering viewpoint, with many illustrations, block diagrams, example circuits and experimental results - many based on the author's personal experience - and use of engineering analytical methods, such as signal flow graphs and Laplace transforms. The author shows how the potential pit-falls in PLL design may be avoided by adopting a rigorous theoretical approach, with almost all results derived from first principles, although mathematics is used for practical relevance rather than academic interest. An important consequence is that the text is substantially self-contained.