Some natural materials emit visible light when bombarded with an electron beam and this is the phenomenon of cathodoluminescence (CL). Carbonate minerals are particularly prone to luminescence, and since ordinary polished thin sections and relatively inexpensive equipment are needed, the technique has become a routine part of carbonate petrography. It is impurities within the carbonate minerals, rather than the major elements, which give rise to most of the visible luminescence. The most important ions affecting luminescence intensity in carbonates are Mn2+ and Fe2+, with the manganese activating luminescence and the iron quenching it. Hence, variations in luminescence intensity usually reflect a variation in the ratio of Mn2+ to Fe2+ in a crystal. Such changes reflect variations in pore-water chemistry or precipitation mechanism. CL studies are a bridge between ordinary petrographic studies and micro-chemical analysis. CL does not reveal absolute concentrations of trace elements, but helps characterise generations of cement and other diagenetic minerals for further analysis. An introduction to cathodoluminescence and its use in carbonate sedimentology can be found in Miller (1988).