In this section the understanding of nanocatalysts, post-aberration corrected electron microscopy is highlighted. A brief introduction to the principles of TEM, HRTEM and STEM is presented, subsequent to which the concept of aberration corrected microscopy is introduced. The theory and history of aberration correction in electron microscopy that has been employed over the previous decades, and that serves to enhance our understanding, in comparison to existing imaging techniques known so far is detailed. The STEM-HAADF technique in aberration corrected microscopes is highlighted and its advantage over conventional TEM imaging techniques is presented. HAADF-STEM technique in combination with various spectroscopic techniques such as EELS and EDS can enhance our atomic understanding of nanocatalysts in an unprecedented way, as never before. This is well demonstrated with various examples of nanocatalysts (on supported substrates as well as unsupported nanoparticles themselves). Both experimental and simulated STEM/HRTEM examples are presented together. These examples include metallic nanoparticles, bimetallic nanoparticles (core-shell and alloys), clusters, multimetallic systems as well as nanowires and nanoplates of layered transition metal chalcogenides. In-situ heating experiments carried out within an aberration corrected electron microscope are also included to present the current state of the art research that is being employed to elucidate and enhance the understanding of nanocatalysts atom-by-atom. Thus with the present capabilities of advanced imaging techniques now common and recently in use, combined with theoretical support there has been a strong platform that has certainly been established in this area of nanocatalysis research.