Metallography, the microscopic study of the structure of metals and their alloys. A piece of metal is composed of very minute grains that change in size and shape as the metal is deformed by stresses or is heated. The size, shape, and arrangement of the grains determine the metal's physical properties (such as strength and ductility). Metallographers study these grains and microscopic flaws such as cracks or bits of unwanted material that may weaken the metal. Metallography is important for the understanding of the behavior of metals. It is also important to metal producers who need to determine the strength and other characteristics of new alloys.

Since even extremely thin pieces of metal are opaque, observations through a microscope must be made with light reflected from the surface of the sample. To be seen, each grain and flaw must reflect light differently.

The surface of the sample is polished until it is as smooth as possible, to remove surface irregularities. The sample is then etched with a solution, such as an acid, that attacks the surface and causes pits and shallow depressions to form. These pits and depressions reflect light at various angles, making some areas—such as grain boundaries and cracks—appear darker than others. Through a microscope, a clear view of the grains and flaws can thus be obtained.

Magnification with an optical microscope is limited to a few thousand diameters. For greater magnifications, an electron microscope—in which electrons are used in place of light—is necessary.