Laser scanning confocal microscopy can be used to image the pore structure of geologic materials in three dimensions at a resolution of 200 nanometers. The technique involves impregnation of the void space with an epoxy doped with a fluorochrome whose fluorescent wavelength matches the excitation wavelength. Optical sections with a thickness of less than 1 micrometer can be sliced from thick polished sections and combined to produce three-dimensional reconstructions. Application of the technique to rocks with porosities from 1 to 20 percent reveals the geometric complexity of the pore space. The technique can also be applied to other brittle solids such as ceramics.