This paper aims at understanding whether producer cooperatives are efficient organisational forms, through the lenses of Transaction Cost Economics. To accomplish this goal I will answer the questions what is a cooperative, why do cooperatives exist, which transactions should producer cooperatives govern, how are they internally organised, and where are the boundaries. This paper offers a comprehensive framework to study producer cooperatives, adopting a comparative institutional point of view, where producer cooperatives are compared with markets, firms and other hybrid types.