  The aim of this text is to provide a clear description of the theory of
Infra-nilmanifolds and their fundamental groups, the almost-Bieberbach groups.
For most of the proofs of the results, we refer to the literature.
Nevertheless, at certain places we offer a somewhat different and new approach
and in these cases we provide full proofs. We have tried to keep the
prerequisites as minimal as possible. We also provide completely worked out
examples, with explicit computations.
