The application of genetic and molecular biological methods to the engineering of proteins, the engine of the biotechnology revolution, has become increasingly dependent on knowledge of protein, nucleic acid and virus molecular structure. Structural information of high precision, essential to this enterprise, can only be obtained through X-ray crystallographic techniques. The quality of this information is directly a function of the properties and degree of perfection of the crucial intermediates, the macromolecular crystals. As a consequence, there is now broad and intense interest in developing new methods, procedures and reagents for the nucleation and growth of such crystals. There is, in addition, an expanded interest in the properties of biological crystals and the use of physical measurements in improving approaches to growing better and larger crystals. Here some of the fundamentals of macromolecular crystal growth will be reviewed, and some current trends in the field remarked upon, including the new initiative to develop a national structural genomics program.