BL Unique resource allowing the reader to progress from the basics right up to the research frontier BL Comprehensive coverage of this new and rapidly developing research topic This new Oxford Logic Guide is the first book to provide comprehensive coverage of a new research area in algebra and model theory. The authors have adopted an algebraic and self-evident point of view rather than a model-theoretic one, making the topic more accessible for pure group theorists. Model theoretical and group theoretical notions are explained in detail, and almost all the known results in the area are included. The authors tailored the book to the needs of the graduate student,with many exercises (with hints) and carefully chosen examples.