  Entanglement monotones, such as the concurrence, are useful tools to
characterize quantum correlations in various physical systems. The computation
of the concurrence involves, however, difficult optimizations and only for the
simplest case of two qubits a closed formula was found by Wootters [Phys. Rev.
Lett. 80, 2245 (1998)]. We show how this approach can be generalized, resulting
in lower bounds on the concurrence for higher dimensional systems as well as
for multipartite systems. We demonstrate that for certain families of states
our results constitute the strongest bipartite entanglement criterion so far;
moreover, they allow to recognize novel families of multiparticle bound
entangled states.
