Interdiffusion microstructures appear as a region on either side of the original interface of contact between two materials. This article outlines the principles used in analyzing various interdiffusion microstructures: binary systems, copper-base systems, nickel-base systems, and silicide-forming systems. The analysis can be helpful in classifying microstructures and in understanding how they change with alloy composition, especially when thermal history is known. The microstructures also help in identifying microstructural artifacts caused by polishing and in recognizing errors in reported heat treating schedules.