We give an introduction into quantum cosmology with emphasis on its
conceptual parts. After a general motivation we review the formalism of
canonical quantum gravity on which discussions of quantum cosmology are usually
based. We then present the minisuperspace Wheeler–DeWitt equation and
elaborate on the problem of time, the imposition of boundary conditions, the
semiclassical approximation, the origin of irreversibility, and singularity
avoidance. Restriction is made to the framework of quantum geometrodynamics.