Ultramicroscopy (UM) is a powerful imaging technique that achieves precise and accurate three-dimensional (3D) reconstructions of intact macroscopic specimens with micrometer resolution. It was developed for specimens in the size range of ∼1-15 mm, such as whole mouse brains, mouse embryos, mouse organs, and Drosophila melanogaster. In UM, the specimen is illuminated perpendicular to the observation pathway by two thin counterpropagating sheets of laser light. UM is closely related to a growing family of comparable microscopy approaches based on light sheet illumination developed in recent years. This article presents an overview of light-sheet-based microscopy and describes the underlying physics of light sheet generation. The assembly of an "ultramicroscope" for investigating fixed chemically cleared tissue is described in detail, and the functions of the essential components, such as mechanics, camera, and objectives, are discussed. Finally, practical applications of UM for studying mouse organs, mouse embryos, and Drosophila adults are described.