Along with increases in empirical information about interspecific mutualisms have come both new and refined questions about them. These questions have spurred diversification in the theoretical approaches being applied to interspecific mutualism. This theoretical literature has become large and potentially confusing, but as a whole is very relevant to answering the current important questions about mutualism. We first present three important questions about mutualisms raised by recent empirical results. (1) What factors control whether interactions become mutualistic or parasitic? (2) Why are highly specialized mutualisms rare and what are the implications of this observation? (3) What is the impact of trophic complexity on the functioning of mutualisms? Second, we highlight results of recent models of mutualism that address at least one of the three questions, and point to potentially rewarding avenues of exploration for these modeling approaches. This review should be useful to both empiricists and theorists as a roadmap to both the variety of theory currently being applied to mutualisms and to results that are in need of additional theoretical and empirical exploration.
Keywords: Biological markets; Mutualism; Parasitism; Prisoner's dilemma; Virulence.