Photochemical activation is a very attractive way to achieve precise spatial and temporal control of the biological action of transition-metal complexes that behave as inactive “prodrugs” in the dark. A significant amount of work has been devoted to metal complexes that act on DNA. In this area, focus has been on ruthenium and rhodium polypyridyl compounds, but copper, iron, cobalt, and vanadium complexes also find increasing application as photoactivable DNA cleaving agents, with excitation sometimes even possible in the near IR region. Most often, the activity of these systems is based on the formation of reactive radical species. Another promising approach is the photochemical generation of covalent DNA binders from inactive precursors, as, for example, by some platinum(IV) compounds. The photolytic liberation of biologically active small molecules from inactive metal complex precursors has also become the target of recent research efforts and complements work on purely organic “caged” compounds. The significant progress made on light-induced liberation of neurotransmitters as well as small molecule messengers like nitric oxide (NO) or carbon monoxide (CO) is also summarized here.