This paper presents a detailed review of the indices found in the literature for voltage stability monitoring.These indices quantify the condition of stability through a scalar magnitude, which can be monitored by network operators, and take measures to avoid a voltage collapse.Unlike other bibliographic reviews, this paper presents the mathematical formulation of each index and an exhaustive classification is carried out considering the following aspects: if the indices use different types of load, if they are based on phasor measurement units -PMU, if they can be calculated in real time, if they consider the operational constraints that arise in a power electrical system, the test systems in which they have been implemented, and if they have been applied in large power systems.Finally, as a result, an analysis that allows to identify work areas, which have been little explored in this topic, specifically those offered by wide area measurement systems WAMS through PMUs, is presented.