A classical and quantum mechanical generalized second law of thermodynamics
in cosmology implies constraints on the effective equation of state of the
universe in the form of energy conditions, obeyed by many known cosmologic
al solutions, forbids certain cosmological singularities, and is compatible
with entropy bounds. This second law is based on the conjecture that causa
l boundaries and not only event horizons have geometric entropies proportio
nal to their area. In string cosmology the second law provides new informat
ion about nonsingular solutions.