This volume collects the lecture notes (ordered alphabetically according to the first author surname) of the talks delivered by the main speakers at the Erice 2007 International School of Crystallography, generously selected by NATO as an Advanced Study Institute (# 982582). The aim of the school was to discuss the state-of-the-art in molecular materials design, that is, the rational analysis and fabrication of crystalline solids showing a predefined structural organization of their component molecules and ions, which results in the manifestation of a specific collective property of technological interest. The School was held on June 7–17, 2007, in Erice (an old town, over 3000 years, located on the top of a Sicilian hill that oversees the sea near Trapani). The school developed following two parallel lines. First we established “where we are” in terms of modelling, 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-lineal optics (NLO), drug delivery, and nanotechnology. In recent years, solid state chemistry and crystal engineering have evolved at the intersection between the top-down and bottom-up approaches towards materials design and fabrication. An ever-increasing number of scientists are learning how to control self-assembly, molecular recognition, and other fundamental processes on the way to achieving ‘tailor-made’ materials, such as crystal nucleation, crystal growth, and polymorphism.