This book has evolved from a set of lecture notes of a course on orthomodular lattices given at the University of Ulm. Most concepts are developed from their very first notions, but in some instances basic set theory and Hilbert space theory may be needed. The text is in general independent of the exercises and supplementary remarks. The book can be used for a general lecture on orthomodular lattices and also for seminars on special geometrical or logical topics. As the first monograph in the field it makes the widely spread results on orthomodular lattices more easily accessible for researchers.