Astrophysical plasmas vary widely in their temperatures and compositions. Some of the most tenuous, coldest, and spatially extensive are the diffuse and dense interstellar clouds, in the latter of which stars are forming. In these regions, including circumstellar shells, ∼130 molecules have been detected with ∼10% being ions. The physical properties and compositions of these regions and the predominant gas phase ionic routes to the molecules are discussed. Examples of recent successes are given and some current problems highlighted.