Nanostructured materials play an important role in today's chemical industry, acting as catalysts in heterogeneous thermal and electrocatalytic processes for chemical energy conversion and the production of feedstock chemicals. Although catalysis research is a longstanding discipline, the fundamental properties of heterogeneous catalysts such as atomic structure, morphology and surface composition under realistic reaction conditions, together with insights into the nature of the catalytically active sites, have remained largely unknown. Having access to such information is however of outmost importance in order to understand the rate-determining processes and steps of many heterogeneous reactions and identify important structure–activity/selectivity relationships, enabling knowledge-driven improvement of catalysts. In the last decades, in situ and operando methods have become available to identify the structural and morphological properties of the catalysts under working conditions. Such investigations have led to important insights into the catalytically active state of the materials at different length scales, from the atomic level to the nano-/micrometer scale. The accessible operando methods utilizing photons range from vibrational spectroscopy in the infrared and optical regime to small-angle X-ray scattering (SAXS), diffraction (XRD), absorption spectroscopy (XAFS), and photoelectron spectroscopy (XPS), whereas electron-based techniques include scanning (SEM) and transmission microscopy (TEM) methods. In this work, we summarize recent findings of structural, morphological, and chemical nanoparticle transformations during selected heterogeneous and electrochemical reactions, integrate them into the current state of knowledge, and discuss important future developments.