Just as astrology became astronomy and alchemy became chemistry through the application of mathematics, descriptive anatomy can be expected to become more and more quantitative in nature. This article describes the basics of stereology, which provides meaningful quantitative descriptions of the geometry of three-dimensional (3D) structures from measurements that are made on two-dimensional (2D) images. With precise mathematical descriptions such as those that can be obtained with unbiased stereological techniques, it will be possible to make concise descriptions of the relationships between structure and function, of the dynamics of structure, and to reassert the importance of quantitative morphology as an essential part of the evaluation of biological tissues.