From interstellar chemistry to prebiotic chemistry: organic matter evolution toward the life The formation and evolution of organic matter starts mostly in dense molecular clouds. These clouds are mainly composed of interstellar grains, including most of the organic matter of 1:he interstellar medium embedded in molecular ices. During the evolution of these grains, this organic matter will undergo many chemical changes (ion bombardment, UV irradiations, and thermal effects) to achieve a true complexity of the organic matrix. In some areas, the cloud will collapse gravitationally on itself to form a "solar nebula" that will evolve into a protostar and potentially to a planetary system like our own. During this evolution, the interstellar grains will agglomerate to form small objects including the original organic matter, which from their evolution around the star can be described as comets or asteroids. These small objects can serve as a reservoir of organic matter for the development of prebiotic chemistry on the surface of terrestrial planets like the Earth, a prelude to the emergence of biosystems as it has indeed been the case on the Earth.