Thermophotovoltaic (TPV) cells are solid-state devices capable of converting the heat radiated by incandescent bodies (named emitters) into electricity using low bandgap semiconductor materials. The characteristics of this technology are very appealing in different applications, such as waste heat recovery, energy storage and portable generators, where high conversion efficiencies can be achieved due to the fact that the emitter and the cells can see each other with high view factors, so it is possible to reflect back to the emitter the low energy photons that do not contribute to photogeneration in the cell. Then, the energy associated to those photons is not considered an energy loss since it can be recycled in the heat source. This recycling process establishes a net radiative heat flux from the emitter to the cells that need to be quantified to determine the TPV conversion efficiency. An overview on the characterization of this efficiency will be presented, together with the record values obtained and the future prospects in the TPV field.