In this book, homogeneous cosmological models whose Hamilton-Jacobi equation is separable are deparametrized by turning their action functional into that of an ordinary gauge system. Canonical gauges imposed on the gauge system are used to define a global phase time in terms of the canonical variables of the minisuperspaces. The procedure clearly shows how the geometry of the constraint surface restricts the choice of time. The consequences that this has for path integral quantization are discussed, and the transition amplitude is obtained for relativistic isotropic models, relativistic anisotropic models (Kantowski-Sachs and Taub) and isotropic string cosmologies. A complete chapter about the application of the deparametrization programme to the usual canonical quantization scheme is also included. This book should be of interest to graduate students and to researchers and lecturers in astrophysics and mathematical physics.