The aims of crystal engineering are the understanding of intermolecular interactions and their application in the design of crystal structures with specific architectures and properties. In general, all types of crystal structures may be considered but this article is limited to organic molecular solids. Because of the molecular basis of organic chemistry, the obvious question arises as to whether there are simple connections between the structures of molecules and the crystals that they form. Answers to such questions may be found through a better and more comprehensive understanding of the interactions that control crystal packing. These interactions include strong and weak hydrogen bonds. Patterns of interactions, such as would be useful in a predictive sense, can be obtained by manual inspection or more rigorously with the use of crystallographic databases. Such patterns are termed supramolecular synthons and they depict the various ways in which complementary portions of molecules approach one another. The identification of synthons is then a key step in the design and analysis of crystal structures. Such ideas are also important in the understanding of phenomena such as biological recognition and drug-enzyme binding. Pattern identification also leads to the possibility of comparison of crystal structures. The use of the supramolecular synthon concept facilitates such efforts and in this regard it may be mentioned that synthons combine topological characteristics with chemical information, thereby offering a simplification that is optimal to drawing such comparisons.