This book is devoted predominantly to one particular way of looking at the evolution of the system in which we describe time changes as transitions from one state to another. This leads in a natural way to the semigroup theory that has been developed in the last fifty years. There are many applications coming from kinetic theory, fragmentation theory, mathematical biology and many other fields, where the assumptions of standard perturbation theory are not satisfied. In this book the authors concentrate on perturbation results that take advantage of the fact that in many applications the operators involved are positive (as only nonnegative data and solutions make sense) and dissipative. The main part of the book is devoted to linking the properties of the generator with the properties of the semigroup, that are important in applications. In the final part of the book the authors discuss applications of the developed theory to a variety of problems ranging from the classical birth-and-death type problems of population dynamics, through fragmentation models in both conservative and mass loss regimes, to kinetic models. The authors also discuss a few miscellaneous applications that do not fit into their theory exactly but nevertheless their treatment draws substantially from it.