Electron microscopes differ from optical microscopes in that they use an electron beam, rather than light, to interrogate a sample. There are many different types of electron microscopes, but the scanning electron microscope equipped with an energy-dispersive x-ray spectrometer (known as SEM/EDX) is perhaps the most commonly used in concrete petrography. SEM/EDX offers the ability to examine surfaces at very high magnifications and obtain in situ or in place chemical analyses of very small regions. When coupled with careful optical petrographic studies the SEM/EDX can be a valuable tool for the concrete petrographer.