Over the last decade, the field of adaptive control has become a major research topic. This book presents a state-of-the-art report on a more specific area: the system classes under consideration which contain linear (possibly nonlinearly perturbed), finite dimensional, continuous time systems which are stabilized by high-gain output feedback. The properties of minimum phase systems and strictly positive real systems are studied in their own right. These results are applied to design simple adaptive controllers involving a switching strategy which is mainly tuned by a one-parameter controller based on output data alone. The control objectives are stabilization, tracking and servomechanism action. In addition, robustness with respect to non-linear perturbations and performance improvements are investigated.