The occurrence of cooperation is one of the greatest challenges for evolutionary biology. The problem is why should an individual carry out a cooperative behaviour that is costly to perform, but benefits other individuals? The theoretical framework of inclusive fitness has proofed extremely successful in explaining the evolution of cooperation across a wide number of taxa ranging from microbes to mammals. Particularly, the investigation of microbial cooperative systems have opened a completely new research area because microbes allow observing the evolution of cooperation in real-time. Furthermore, many cooperative traits are involved with virulence in infections of humans, and have therefore medical relevance. Consequently, the research in this area is highly interdisciplinary connecting the fields of evolutionary biology, microbiology and medicine. This proposal focuses on the production of siderophore molecules in the opportunistic human pathogen Pseudomonas aeruginosa. Iron is a major limiting factor for bacterial growth and in response to iron deficiency, P. aeruginosa releases siderophore molecules into the local environment to scavenge insoluble iron, making it available for bacterial metabolism. Siderophore production is a cooperative behaviour as it can provide a fitness benefit to neighbouring cells, which can take up iron bound to siderophore produced by others. This proposal uses cooperative siderophore production to study: (i) the properties of siderophore molecules as a cooperative good and virulence factor, and assess the fitness consequences for cooperative wildtype and non-cooperative mutant strains in mixed cultures in vitro and in animal host models; (ii) the dynamics of cooperative and non-cooperative strains under different environmental conditions as they may occur in nature; and (iii) the adaptive responses of cooperators to the presence of non-cooperative strains at the behavioural and genetic level in an experimental evolution set up.