Stereology is a branch of applied mathematics used for the three-dimensional analysis of organs and materials from two-dimensional measurements. Stereologic methods are versatile, easily understood, and simply applied. This paper provides anatomists with a practical guide to stereologic techniques and concepts and to ancillary morphometric methods. These methods allow investigators to calculate volume, surface, and numerical densities; make shape determinations; and calculate size distributions for spheres and rotatory ellipsoids. Other topics covered in this article include sampling methods, correction factors for shrinkage and section thickness, index of folding, automated and semiautomated image analysis, and three-dimensional reconstruction from serial sections.