Metal nanoparticle (NP) catalysts are the key enablers of durable chemical technologies, opening the road towards carbon circularity. The atomic structure of NPs, and in particular the surface configuration, governs reactivity where adding or removing a single atom can have profound impact. To date, however, state-of-the-art characterization tools do not allow facile investigation of the 3D NP structure under relevant (reaction) conditions. This impedes foundational understanding of NP formation and functioning, crucial to tailoring the properties and performance of metal catalysts. Herein, we will explore sophisticated X-ray characterization tools to gain access to the in situ/operando 3D NP structure under relevant gas conditions, both during NP formation and functioning.