There are very few quantum systems that can be solved exactly. In most cases, one has to use some approximate method. One of the most important approximate methods is perturbation expansion. The main idea of this method is simple: if one knows the solution of some parent system, one can find the solution of the “perturbed” system. It is assumed that the system does not differ from the parent system very much, in which case “perturbation” is small. Next one expresses eigenvalues and eigenstates of the perturbed systems as a power series expansion in terms of the eigenvalues and eigenstates of the parent system. Hence in order to obtain the solution of the perturbed system, one has to know the solution of the parent system. This book offers examples and solutions to this fascinating topic.