Determination of the structural character of a material, whether massive in form or particulate, crystalline or glassy, is a central activity of materials science. The general approach adopted in most techniques is to probe the material with a beam of radiation or high-energy particles. The radiation is electromagnetic in character and may be monochromatic or polychromatic: the electromagnetic spectrum conveniently indicates the wide choice of energy that is available. The microscope, in its various forms, is the principal tool of the materials scientist. The magnification of the image produced by an electron microscope can be extremely high; however, on occasion, the modest magnification produced by a light stereomicroscope can be sufficient to solve a problem. In practical terms, more importance is given to resolution than magnification, that is, the ability of the microscope to distinguish fine detail. This chapter examines the principal ways in which light, X-rays, electrons, and neutrons are used to explore the structure of metals. Although the prime purpose of microscopy is to provide qualitative information on structure, many complementary techniques are available that provide quantitative data on the chemical and physical attributes of a material.