The book is devoted to the theory of gradient flows in the general
framework of metric spaces, and in the more specific setting of the
space of probability measures, which provide a surprising link between
optimal transportation theory and many evolutionary PDE's related to
(non)linear diffusion. The book originates from lectures by L. Ambrosio
at the ETH Zürich in Fall 2001. It contains new results that have never
appeared elsewhere. The book has been substantially extended and revised
in cooperation with the co-authors. Particular emphasis is given to the
convergence of the implicit time discretization method and to the error
estimates for this discretization, extending the well established theory
in Hilbert spaces. The book is split in two main parts that can be read
independently of each other.