In the recent years, nanotechnology has rapidly evolved as promising toolbox for many applications, including sensing and drug delivery. Nanotechnology aims at forming man-designed two-dimensional and three-dimensional structures in the nanometer scale using e.g., the self-assembly properties of smaller building blocks such as DNA and RNA. The visualization and structural characterization of these nanostructures do not only provide evidence for the correct formation of the desired shapes, but can also contribute to a better understanding of their formation and functionality. Transmission electron microscopy offers the possibility to directly visualize the individual nanostructures. The vitrification of the sample by using the plunge-freezing method and subsequent electron cryomicroscopy (cryo-EM) provides in-solution snapshots of the nanostructures under cryogenic conditions and thus preserves the close-to-native structure of the particles. Here, we describe the plunge-freezing and other sample preparation protocols such as negative staining and cryo-negative staining as well as the various imaging and image processing methods, including electron crystallography, electron tomography, and single-particle EM. Typical example applications are provided together with a discussion of benefits and shortcomings of these approaches. We also discuss how deviations from an ideal symmetry and structural heterogeneity, in general, can limit the resolution. Finally, we suggest that nanotechnological approaches may not only offer new applications in the field of nanomaterial science and nanomedicine, but may also emerge as tools for structural biology and structure-related biomedical research. Microsc. Res. Tech., 2011. © 2010 Wiley-Liss, Inc.