Discard of unwanted catches are common in European fisheries, but reducing or banning this has been given high priority in the proposal for the reform of the Common Fisheries Policy. Although many technical regulations have been introduced to limit unwanted catches, there is little understanding of the underlying socio-economic and institutional incentives causing discard at the fisher level. The paper presents an approach which views discards as a result of decisions made both on deck and at earlier stages of the fishing planning and implementation process. Decisions made by fishers resulting in a more selective fishery are considered “selective behaviour”. It is argued that fishing practices are institutionally embedded within three institutional spheres: “state”, “market”, and “community”, which together with “natural conditions” create incentives and frameworks for discard and selective behaviour. A comprehensive list of factors which may influence discards and selective behaviour is developed and applied to three case studies—all trawl fisheries—in Denmark, Greece, and England. The paper discusses cross-case findings of how the identified factors may create drivers for discard. Finally, a refined list of factors is presented in a tree structure and the usefulness of the list as a tool for analysing drivers for discard and selective behaviour, in a context of developing mitigating measures, is discussed.