We present results on optical detection of individual dye molecules diluted at low concentration iii crystals and in polymers. There is now no doubt about the feasibility of such experiments which can easily allow the study of local environments. This paper is devoted to our contribution from the beginning to this spectroscopy domain in which several papers have recently appeared. We also report results about Stark effect on single molecule in a solid. In this particular case we choose to apply an electric field on a non-centrosymmetric molecule in a disordered system.