Mutualistic microbial symbioses are one of the key innovations in the evolution of biological diversity, enabling the expansion of species' niches and the production of sophisticated structures such as the eukaryotic cell. For some of the best-studied cases, we are beginning to have network models of symbiotic metabolism, but this work is in its infancy and has not been developed with an evolutionary perspective. However, theoreticians have long been interested in how these symbioses arise and persist and have applied modelling approaches from economics, evolution, ecology, and sociobology to a number of fundamental questions. We provide an overview of these questions, followed by specific modelling examples. We cover economic game theory, including the Prisoner's Dilemma, the Snowdrift game, and biological markets. We also describe the eco-evolutionary framework of adaptive dynamics, inclusive fitness, and population genetic models. We aim to provide insight into the strengths and weaknesses of each approach and into how current evolutionary methods can benefit an understanding of the mechanistic basis of host-symbiont interactions elucidated by molecular network models.