Planar micro-cavity light emitting diodes (MCLEDs) are formed by a vertical Fabry-Perot cavity containing an active layer. In the "weak coupling" regime, the fundamental mode of the confined dimension of the cavity corresponds to the emission peak wavelength of the active medium. The cavity interference effect selects the emitted mode, increases its power at the expense of the other modes and modifies its spatial distribution. Consequently, MCLEDs feature an increase of the extraction efficiency, of the directionality of the emission and a narrowing of the emission spectrum. These advantages over standard light emitting diodes make the MCLEDs very attractive for a pplications such as lighting, interconnects, gas sensing and for fiber-optic communications at l=0.65 µm; and from l=0.85 µm to l=1.5 µm. My research has concentrated on improving the extraction efficiency and the bandwidth of 0.65 µm and 1.3 µm MCLEDs. This experimental and theoretical work was supported by the European Esprit project, SMILED and an Irish strategic project.