Intermediate band (IB) solar cells aim to exploit in solar cells the energy of below bandgap energy photons. They are based in a material that, in addition to the conventional conduction and valence bands, has an electronic band (named intermediate band) located inside the bandgap and separated from the conduction and valence band by a null density of states. The theoretical limiting efficiency of these cells (63.2 % at maximum concentration) is equivalent to a triple junction solar cell but requiring a single material instead. Several approaches are being followed worldwide to take to practice this concept that can be divided into two categories: quantum dots and bulk materials. This paper reviews the main experimental results obtained under both approaches.