In algebraic topology some classical invariants - such as Betti numbers
and Reidemeister torsion - are defined for compact spaces and finite
group actions. They can be generalized using von Neumann algebras and
their traces, and applied also to non-compact spaces and infinite
groups. These new L2-invariants contain very interesting and novel
information and can be applied to problems arising in topology,
K-Theory, differential geometry, non-commutative geometry and spectral
theory. The book, written in an accessible manner, presents a
comprehensive introduction to this area of research, as well as its most
recent results and developments.