Metal nanoparticles are materials of great scientific interest with a vast number of applications. In catalysis, also including electrocatalysis, the use of metal nanoparticles has provided very remarkable advances, and they are being applied in many reactions to obtain enhanced catalytic activity and/or to modify their catalytic selectivity. Nevertheless, in order to optimize/modulate these (electro)catalytic properties, it is frequently required to have a fine control of their surface. In this chapter, different surface treatment strategies for metal nanoparticles and their effects from a catalytic point of view are reviewed. Finally, a brief section devoted to the use of metal nanoparticles as platforms for biomolecules attachment for biosensing and bioanalytical applications is also included.