During the last six decades electron microscopy (EM) has been essential to ultra-structural studies of the cell to understand the fundamentals of cellular morphology and processes underlying diseases. More recently, electron tomography (ET) has emerged as a novel approach able to provide three-dimensional (3D) information on cells and tissues at molecular level. Electron tomography is comparable to medical tomographic techniques like CAT, PET and MRI in the sense that it provides a 3D view of an object, yet it does so at a cellular scale and with nanometer resolution. Electron tomography has the unique ability to visualize molecular assemblies, cytoskeletal elements and organelles within cells. The three-dimensional perspective it provides has revised our understanding of cellular organization and its relation with morphological changes in normal development and disease. Cryo-electron tomography of vitrified samples at cryogenic temperatures combines excellent structural preservation with direct high-resolution imaging. The use of cryo-preparation and imaging techniques eliminates artifacts induced by plastic embedding and staining of the samples is circumvented. This review describes the technique of cryo-electron tomography, its basic principles, cryo-specimen preparation, tomographic data acquisition and image processing. A number of illustrative examples ranging from whole cells, cytoskeletal filaments, viruses and organelles are presented along with a comprehensive list of research articles employing cryo-electron tomography as the key ultrastuctural technique.