The activation of small molecules has a significant impact in biology, medicine, industrial catalysis and environmental protection. Two molecules, especially, oxygen and nitric oxide have attracted considerable interest for many years. Reactive oxygen species (ROS) and reactive nitric oxide species (RNOS) may be generated thermally or photochemically in systems consisting of metal compounds. In homogeneous systems metal species serve as a coordination and/or electron (eventually energy) transfer centre. In heterogeneous systems small molecules can undergo adsorption followed by electron or energy transfer processes with semiconductor participation. This review presents recent trends in studies on the metal assisted processes in which activated forms of NO or O_{2} are formed and play a key biological and medical role.