This book brings together interdisciplinary contributions ranging from applied mathematics, theoretical physics, quantum chemistry and molecular biology, all addressing various facets of the problem to connect the many different scales that one has to deal with in the computer simulation of many systems of interest in chemistry (e.g. polymeric materials, biological molecules, clusters, surface and interface structure). Particular emphasis is on the "multigrid technique" and its applications, ranging from electronic structure calculations to the statistical mechanics of polymers.