Research into the physical properties of organic solids, especially those containing conjugated pi-electron systems, has developed into an active and attractive sub-area of solid-state physics over the last few decades.There are several reasons for this development. First of all, there is the enormous diversity of properties typical of organic solids, such as long-distance energy conduction via excitons without electric-charge transport. With the powerful methods of organic chemistry, it is possible to vary these properties over wide ranges with ''tailor-made'' molecules. Secondly, new applications are under development, such as organic light-emitting diodes and novel molecular electronics, which supplement electronic components based on inorganic semiconductors. Finally, organic solids represent a link between traditional physics and biological physics: Organic solid-state physics has made important contributions to the clarification of the elementary processes of photosynthesis, for example.Organic Molecular Solids has been written for graduate students and researchers, but will also be an interesting and valuable information source for all physicists, physical chemists and chemists wishing to learn more about the fascinating variety of organic solids. This is an introduction to the fundamentals of this topic, featuring detailed references, reading lists and problems.