Nanostructured surfaces and colloids with designed optical functionalities - such as plasmonic and photonic nano and metamaterials - allow efficient harvesting of light at the nanoscale. Exploiting light-driven matter excitations in these artificial materials opens up a new dimension in the conversion and management of energy at the nanoscale [1-4]. One example are plasmonic and photonic catalysts, that can indeed confine solar energy into molecular regions [4-11].In this talk, I will present our recent efforts in order to understand the intricate realm of losses in plasmonic nanoparticles by studying this issue at the single-particle level. I will present a series of studies trying to correlate plasmonic photocatalytic activity with reactivity, temperature and field at the nanoscale. Our fundamental understanding of light-matter interaction in these systems can hopefully contribute to the rational design of future nanocatalysts; influencing diverse applications such as photocatalysis and temperature management, among others [1-11].