Complex responses have been observed in a wide range of chemical and engineering systems: In well-stirred and unstirred solution-phase, in gels, in heterogeneous catalysis and dissolution reactions, in gas- and solid-phase combustion, widely in biological systems, accompanying phase transitions, in atmospheric kinetics and even in interstellar dust clouds. Complex behaviour does not, however, imply a necessarily complex underlying chemical mechanism. The appropriate feedback mechanisms, built on chain-branching, autocatalysis or self-heating, arise quite commonly in Nature. In many instances, it is appropriate first to attempt to find reduced mechanisms giving a semi-quantitative fit to observed responses. An example of such an approach applied to a model for complex oscillations of species concentrations in the mesosphere is presented, and the reduced model used to investigate the effects of longitudinal mixing in this system. Once a general understanding of a particular reaction system has been obtained, more detailed questions can be addressed. This is exemplified by a study of the development of three-dimensional scroll waves in the Belousov-Zhabotinsky reaction. Such detailed information for a specific reaction also informs the general understanding of the class of 'excitable media'.