With increasing environmental concerns worldwide as well as the scarcity and cost of metallic catalysts, the use of nanoparticles is a novel way to exploit resources more efficiently and minimize waste. The properties of metallic nanoclusters are highly sensitive to the size, shape, support and catalytic conditions. This creates a complex picture but, at the same time, offers the possibility of achieving the greatest performance of these materials, as well as stability against deactivation. There are many opportunities in the design of such nanocatalysts but we are just starting to understand their true nature. In this talk we will analyze different factors influencing the catalytic properties of Pt-based nanocatalysts. Through the understanding and manipulating their electronic structure we will predict optimal sizes and compositions in freestanding and MgO-supported Pt nanocatalysts. Finally, size and composition effects will be explored in the WOx/ZrO2 acid catalyst.