Three-dimensional (3D) characterization of materials at the nano- and meso-scale has become possible with transmission and scanning transmission electron microscopes (S/TEM). These 3D datasets require advanced software tools for analysis and visualization. The Tomviz application is tailored for visualizing electron tomography data. It can utilize the large quantities of memory and processing resources required to render, manipulate, and analyze voluminous 3D tomograms. The platform provides a robust graphical interface where objects can be rendered as shaded contours or volumetric projections. Multiple datasets, colormaps, and other visualization settings can be used in combination and these objects can be rotated, sliced, animated, and saved as image or video files. Data collected can be further analyzed through histograms, multicorrelative statistics, multiple filters, and user-customized Python scripts. Tomographic reconstructions of experimental data are also possible within Tomviz. We recommend a 64 bit Windows, macOS, or Linux operating system with 8+ GB of RAM and a discrete NVIDIA graphics card for best performance.