We describe recent studies on photothermal effects using colloidal nanoparticles. Metal nanoparticles efficiently generate heat in the presence of electromagnetic radiation. This process becomes strongly enhanced under plasmon resonance and also depends on the shape and the organisation of the nanoparticles. In particular, the amount of generated heat and temperature increase depends on the number of nanoparticles in a complex. Metal nanoparticles may induce phase transformations when they are in thermal contact wih a polymer or a solid matrix, such as ice. This suggests new possibilities for measuring heat release at the nanoscale.