Mutualism, a reciprocal beneficial interaction between organisms, involves ecological and evolutionary processes occurring at several scales of organization. For many decades mutualism was the stepchild of ecology, neglected, malnourished and not studied theoretically because the prevailing paradigm was ‘nature red in tooth and claw’. Positive interactions appeared to be both more difficult to accept on theoretical grounds and to test experimentally than negative interactions. In particular, trying to understand the conditions for stability and long-term outcome of positive interactions was (and still is) a great challenge. Now it is appreciated that there exists a continuum in the interactions between individuals belonging to different species, like ants and their insect partners, and this raises several interesting questions. For example:

• How can the different life histories of ants and their partners be merged so that interactions become beneficial?

• How does the abundance of the partners affect the strength of these interactions?

• What are the population and community consequences of mutualistic relationships for the interacting partners and indirectly affected species?

• Do mutualistic interactions affect species diversity?

• How does the environment affect the outcome and stability of these associations?

• How can the different partners of ants coexist in local and regional communities?

• How can mutualists persist in the face of exploiters?

In this book we aim to explain the underlying mechanisms of the dynamics of these associations by adopting a view that is not ant-centred, because the selection pressures of such associations are likely to affect both partners.