This volume is the proceedings of the Ninth Taniguchi Symposium on the Theory of Condensed Matter. It presents the basic ideas of quantum statistical Monte Carlo methods, including generalizations of the standard methods, and discusses the important problem of convergence. In addition, some promising powerful new methods are proposed, for example, a transfer-matrix method. Further contributions describe specific applications to quantum spin and fermion systems at finite temperatures, such as the treatment of randomness and frustration in a triangular antiferromagnet, the investigation of critical properties of spin chains, the study of spin glasses and the simulation of polaron and bipolaron systems. In particular, some new information is given about frustrated quantum spin systems and the dense Kondo problem.