Evidence from a variety of space experiments confirms that gravity does, in some cases, have a significant effect on the crystallization of macromolecules. This has been demonstrated by numerous investigators employing a wide range of instruments and proteins. In the absence of gravity, convective transport of solute is eliminated and pure diffusion predominates. This has the consequence of producing local regions of reduced supersaturation thereby enhancing conditions for optimal growth. In addition, sedimentation of microcrystals and large impurities onto the surfaces of growing crystals, as well as direct contact between crystals, is minimized. The mechanisms for biological macromolecule crystallization, the unique features of their growth, and techniques for growing and characterizing crystals of proteins and viruses are described. The instruments currently available and under development for macromolecular crystal growth and its study in microgravity are reviewed.