With the changes that have taken place in society and the economy, large companies are joining forces to dominate the market more and more. Small and medium-sized companies act together in solidarity as a way of ensuring their competitiveness and survival, producing new organisational formats of the network type. Despite studies on cooperation networks, little attention has been paid to leadership in these organisations. This book, the result of a master's research project, therefore deals with the subject of leadership in SME associative co-operation networks. To this end, the study was based on quantitative, qualitative and exploratory research and the method used was the case study. The empirical field of investigation was a co-operation network belonging to the retail segment, in the building materials sector, called Rede Centersul, located in Mato Grosso, Brazil. The findings showed that leadership in the network is made up of formal and informal structures and that its performance is important for the development and maintenance of the co-operation network, since it helps to increase trust and the growth of social capital.