The degradation of metal clusters and nanoparticles under electrochemical control is a major challenge to overcome in many applications, for example, to develop efficient, stable, and cheap energy devices. In particular, Pt nanoparticles are catalysts in polymer electrolyte fuel cells with relevance to the automotive industry. Understanding the degradation mechanisms of metal clusters and nanoparticles is important to develop effective mitigation strategies and optimize, for instance, the lifetime of electrocatalytic devices. The main degradation mechanisms in such devices are presented: migration coalescence, metal dissolution, electrochemical Ostwald ripening, and particle detachment. Understanding the dynamic and often complex degradation pathway has been possible through the development of investigation techniques, which are highlighted as well.