The physical and chemical processes at work in the interstellar medium (ISM) play a central role in the regulation of the star formation process and thus on the evolution and shaping of galaxies. Conversely, stars, through their radiation fields, winds and supernova explosions profoundly affect the physical and chemical properties of the ISM and contribute to its evolution and composition. The ISM presents an extraordinary molecular richness: to date almost 200 molecules have been detected. These molecules range range from simple polyatomic species to complex molecules containing more than 10 atoms. The type of environments in which these molecules are observed cover a broad range of physical conditions, varying from low density and irradiated regions to very dense and opaque gas.

Interstellar matter is thought to evolve from diffuse interstellar clouds to high density regions (i.e. dark clouds). The resulting increase in gas column density results in increased shielding from photodissociation, leading to the development of a rich molecular chemistry in both the gas phase and at the surface of the grains. Because molecules play an important role in the thermal balance of the ISM and because they give us a unique access to its physical conditions, understanding the chemistry at work is mandatory if we are to understand its physical evolution.

In this talk I will review our current understanding on the physical evolution from the most diffuse to the densest part of the ISM. I will discuss the chemistry associated with this evolution for both the gas and at the surface of the grains. Finally I will discuss how physical conditions in the past can be important when establishing the molecular content of dense clouds.