The importance of protein crystals for structural studies of biological macromolecules, peculiarities of protein crystals, and their growth under terrestrial and microgravity conditions are considered. The reasons for the better quality of protein crystals grown in space are discussed. The history of protein crystallization experiments under microgravity conditions from the very beginning to the present is briefly reviewed. The contribution of space experiments to the theory of the protein crystallization, the development of new crystallization methods and the design of new crystallization devices is highlighted. The revelation of structure–function relations for some proteins based on their 3D structures solved using crystals grown under microgravity conditions within the framework of the JAXA/PCG project is presented and arguments in favour of space experiments are given.