This volume collects the state of the art in molecular materials. It collects the lecture notes of a series of lectures given by some of the best specialists in the field at the 2007 Erice International School of Crystallography –and also a NATO–ASI course–. The school developed along two parallel lines. First we stablished "where we are" in terms of modeling, design, synthesis and applications of crystalline solids with predefined properties. Second, we attempted to define current and possible futuristic lines of development in the quest for novel molecule–based materials with potential applications in magnetism, conductivity and superconductivity, non–linear optics (NLO), drug delivery, and nanotechnology.