As an easy technique for phase determination in X-ray analysis of macromolecular crystals, the molecular replacement method is widely used. Available computer programs are highly sophisticated to user friendly so that users can use them without knowledge on the theoretical backgrounds. To develop the technique to more powerful tool, as well as to resolve individual problems in special cases, it is essential to understand the details of the backgrounds. Here the theories of the method with the historical backgrounds, its availability and capability, practical usage and trouble shooting will be described.
