In his work on rings of operators in Hilbert space, the author discovered a new mathematical structure that resembled the lattice system Ln. In characterizing its properties, John von Neumann founded the field of continuous geometry. This book, based on von Neumann's lecture notes, begins with the development of the axioms of continuous geometry, dimension theory, and - for the irreducible case - the function D(a). The properties of regular rings are then discussed, and a variety of results are presented for lattices that are continuous geometries, for which irreducibility is not assumed.