Ants play a central role in shaping terrestrial ecosystems, and have repeatedly evolved complex relationships with other organisms, including insects, plants and microbes. These interactions range from parasitisms to mutualisms. I will discuss three examples: the association between ants and caterpillars of butterflies in the family Lycaenidae, the interaction between ants and their gut microbiota, and the relationship between ants and ant plants found on the savannahs of East Africa. Collectively, these interactions illustrate how symbiosis can be a creative force in facilitating adaptive radiation through the acquisition of novel capabilities that permit exploitation of new ecological niches, and at the same time pose constraints on the evolution of both partners through the need to coordinate opportunities for interaction as well as through the population consequences of increased specialization.