The growth of molecular complexity in the Universe

A review is presented of the chemistry of the early Universe and the formation of the first stars. The critical role of the formation and destruction of molecular hydrogen in determining their initial mass is demonstrated. A qualitative account is given of the transformation of astrochemistry that occurred in response to the introduction into the Universe of metals. The diagnostic value of astrochemistry is illustrated by its use to infer the Galactic ionization rate. The chemistry of the interstellar medium is contrasted with that of the atmospheres of planets and recent studies of giant extrasolar planets are summarised.