Various theories to account for clouded plagioclase are reviewed. Observations indicate that the particles causing this effect may consist of different minerals, and that many minerals, besides plagioclase, may show clouding. It is suggested that in clouded minerals there are minute surfaces of physical discontinuity which provide adequate passages for diffusion of material into and out of the crystals. In intermediate plagioclases these surfaces may consist of internal phase boundaries in the unmixed feldspar. Slight clouding is probably due to exsolution of iron present in the feldspar lattice at the time of its formation, but more intense clouding is believed to be the result of the migration of iron and other elements into the crystal after its formation. The geological significance of clouded plagioclase is discussed and it is shown that such feldspars cannot be used as sole criteria for thermal metamorphism.