Photovoltaic power generation systems have been globally installed in recent years and they are today widely adopted as one of the most inherent and cost-competitive renewable energy sources. Consequently, photovoltaic plant monitoring is becoming a crucial task to guaranteeing a stable and reliable operation in addition to effective cost management. This paper presents a detailed review of different monitoring systems reported in the literature, which strongly depend on the photovoltaic installation topology and output power range. This involves various monitoring techniques and the monitoring system concerns, opportunities and challenges.