This special issue on Optimisation Solutions in Systems showcases recent developments in a thrilling and undoubtedly practical research field. Indeed, optimisation is at the heart of many scientific and industrial processes, yet it also harbours some important theoretical and practical limitations not to be ignored. Ranging from areas as diverse as medical applications to circuit design, the papers included in this special issue illustrate the intimate interdependence between the problem under consideration in each case and the method or metaheuristic used to tackle it - and simultaneously, they provide enhanced optimisation algorithms that will help to address other problems in the future.