Cryo-electron tomography (cryo-ET) is a cutting-edge technology providing three-dimensional in situ ultra-structural information of macromolecular machineries,organelles,and eukaryotic cells in their native environment at an unprecedented level of detail.Cryo-ET enables the direct observation of dynamic macromolecular architectures of bio-samples in their naturally occurring physiological state,without any harmful artifacts introduced by heavy metal staining,dehydration,and chemical fixation,which occur in traditional transmission electron microscopy.Over decades,cryo-ET has been providing insights into numerous aspects of cellular biology by revealing the pristinely preserved ultra-structures of different cellular components comprising the crowded and complex environment of the cell,thus,bridging the gap between cellular biology and structural biophysics.In this paper,we review the fundamentals of this technique,its recent advances in optics,detection devices,and computational algorithms.The enhancement of our understanding of structural cellular biology by combining these improvements,when integrated with other methods,such as cryo-focused ion beam milling,correlative light and electron microscopy,is discussed via a few examples from research groups worldwide.We also believe that cryo-ET applications in cell biology continue to provide fundamental insights into the field,revolutionizing structural biology itself.