The problem is treated of reconstructing the spatial structure of biological macromolecules and their aggregates (e.g., in viruses and crystals) from electron micrographs, which are two-dimensional projections of these three-dimensional objects. Potentialities of the physical methods (optical diffraction, filtering, and holography) in interpreting electron micrographs are described. The fundamentals are given of the mathematical theory of three-dimensional reconstruction from projections: the Fourier method, the algebraic methods, and the analytical methods. Applications are reviewed of the three-dimensional reconstruction methods in studying a number of objects: protein crystals, helical structures made of globular proteins, bacteriophages, and spherical viruses.