Molecular clouds contain most of the mass of gas in interstellar space and are the source of matter from which new stars are formed. The clouds can be studied by means of the radio emission in rotational transitions of the molecules that they contain. We describe the chemical routes by which these molecules are formed, and their ultimate incorporation into icy mantles frozen onto dust grains. The role that chemistry plays in tracing and controlling processes in star-forming regions and in near-stellar environments is briefly indicated.