This paper demonstrates the applicability of quantitative interference microscopy to the study of minerals and rocks in transmitted light. For readers who are not familiar with interference microscopy, the principle, the detectability and measurement of interference fringe shifts, and the precision of its use are discussed. The following are some of its applications: (1) the far superior ease and precision of determination of the refractive indices of minerals over the traditional Becke line method (there is no need to estimate the match point, but instead it measures the difference of the index between the mineral and immersion medium, hence minimizes the operator’s error; a precision of ±0.0002 can be readily obtained); (2) the possible identification of micron-size inclusions; (3) identification of minerals in thin sections by the zero-fringe method; (4) display of chemical variations in minerals; and (5) the study and possible identification of microstructures, such as twinning, zoned crystals, rimmed crystals, exsolution, and various types of lamellar microstructures produced by static or shock deformation. Procedures for measuring interference fringe shifts in various applications are included in an appendix. Complementary use with X-ray diffraction, electron-microprobe analysis, and transmission electron microscopy and electron diffraction gives the proper perspective of the role of interference microscopy in mineralogic and petrologic investigations.