The GRE Subject Tests are intended to measure your knowledge of specific subject matter. Subject Tests are available in eight disciplines: biochemistry, cell and molecular biology; biology; chemistry; computer science; literature in English; mathematics; physics; and psychology. These tests are given in paper–and–pencil format three times per year, in September, October and April worldwide.
These tests are given separately from the GRE General Test and require additional fees of US $ 150. Check with the graduate schools to which you are applying to determine whether any of these extra tests are required. You must take these tests only if a university asks for it. Moreover, it is mainly taken by those students who wish to specialize in explicit areas like Math, Physics, etc.