At first sight, the interstellar medium presents a very hostile environment to the formation and survival of complex molecules. As we have seen, it is crisscrossed by violent shocks and pervaded by intense UV and X-ray radiation fields, which can easily tear apart delicate molecular bonds. Even the composition of the interstellar medium presents a problem to the formation of complex molecules, since the gas is mostly hydrogen and helium. Reactive species are simply trace elements, and direct collisions between them rare. Nonetheless, astrochemistry turns out to be a rich and complex field. In cool regions in the tails of shocks, gas-phase reactions can occur on interestingly short timescales. Within dense clouds, UV photons and cosmic-rays help moderate a whole series of chemical reactions. Migrating molecules can meet and react together on the surfaces of grains and very complex molecular species can be built up. In this chapter, we will simply get a taste for these phenomena. To find out more, the recent book by Emma Bakes (1997), and the reviews by Dalgarno (1987) and Genzel (1992) are strongly recommended (see notes on this chapter).