This book presents a systematic and unified approach to the nonlinear stability problem and transitions in the Couette-Taylor problem, by the means of analytic and constructive methods. The most "elementary" one-parameter theory is first presented with great detail. More complex situations are then analyzed (mode interactions, imperfections, non-spatially periodic patterns). The whole analysis is based on the mathematically rigorous theory of center manifold and normal forms, and symmetries are fully taken into account. These methods are very general and can be applied to other hydrodynamical instabilities, or more generally to physical problems modelled by partial differential equations. Non-mathematician readers can skip the mathematically "hard" parts of the book and still catch the ideas and results. This book is primarily intended for graduate students and researchers in fluid mechanics, and more generally for applied mathematicians and physicists who are interested in the analysis of instabilities in systems governed by partial differential equations.