Since 2012, the 3D Hybrid Organic Perovskites of formula CH3NH3PbI3 and their derivatives represent a “material breakthrough” for photovoltaïcs: in only 3 years, the efficiency of 3D HOP-based solar cells has progressed from 12% to more than 20%. These HOP molecules appear also very attractive for light-emitting devices such as electroluminescent diodes and lasers. Improvement of the performance of perovskite solar cells and optoelectronic devices may benefit from a better understanding of the intrinsic photophysics of materials. However, the properties of films are very dependent of the grain structure. We will focus here on the synthesis of HOP single crystals and we will study their optical properties. The optical properties of a millimetric CH3NH3PbI3 single crystal are compared to those of thin polycrystalline films. In particular, the main feature in single crystals is the appearance, in the low temperature luminescence spectra, of a sharp emission line (FWHM ~ 5 meV) at high energy which is attributed to the free exciton signature (figure 1). Experiments performed as a function of temperature confirm the existence of a strong electron-phonon coupling. We highlight the extremely high stability of the single crystals compared to the thin layers: defects and grain boundaries are then thought to play an important role in the degradation mechanism.