Materials used for photocatalytic overall water splitting (POWS) are typically composed of light-absorbing semiconductor crystals, functionalized with so-called cocatalytic nanoparticles to improve the kinetics of the hydrogen and/or oxygen evolution reactions. While function, quantity, and protection of such metal(oxide) nanoparticles have been addressed in the literature of photocatalysis, the stability and transients in the active oxidation-state upon illumination have received relatively little attention. In this Perspective, the latest insights in the active state of frequently applied cocatalysts systems, including Pt, Rh/Cr2O3, or Ni/NiOx, will be presented. While the initial morphology and oxidation state of such nanoparticles is a strong function of the applied preparation procedure, significant changes in these properties can occur during water splitting. We discuss these changes in relation to the nature of the cocatalyst/semiconductor interface. We also show how know-how of other disciplines such as heterogeneous catalysis or electro-catalysis and recent advances in analytical methodology can help to determine the active state of cocatalytic nanoparticles in photocatalytic applications.