In the past, it has been conventional to view interstellar molecules as probes of cold, low-energy environments. We now find many instances of molecules flourishing in the presence of energetic radiation, high temperatures and superthermal components of the molecular speed distribution. This study centres on reactive molecular ions as specific tracers of such harsh physical environments. Reactive ions such as H2+, CH+ and CO+ constitute a special class because they are destroyed on virtually every collision with the most abundant species H, H2 and e-. The abundance and excitation of CH+ and CO+ are discussed with reference to recent observations of nebular boundary layers. The hydrogen molecular ions H2+ and H3+ achieve high abundances in X-irradiated molecular gas, such as that in the hypothesized molecular tori surrounding active galactic nuclei. A population inversion is predicted in the (J, K)=(4, 4)→(3, 1) transition of H3+ and the conditions required for maser amplification are discussed.