Photovoltaic (PV) systems operating in real conditions of work are very often subject to several faults that may lower significantly the produced energy and shorten their availability. Therefore, powerful and trusted fault detection procedures are necessary to enable early maintenance and avoid excessive energy losses. The large increase in PV power installed worldwide in recent years, especially in systems connected to electricity distribution networks: Grid-connected PV systems, has led to the development of strategies and tools for automatic supervision of these systems to detect faults and diagnose the source of these failures. This chapter presents a review of most relevant existing methodologies applied in fault detection and diagnosis of PV systems.