Mutualistic interactions are an integral component of the earth’s biodiversity. Despite their importance, theory on mutualistic interactions has lagged behind that for competitive and consumer-resource interactions. Moreover, existing theory on mutualisms focuses mostly on above-ground (e.g., plant-pollinator/seed disperser) interactions. Below-ground mutualisms such as plant-mycorrhizal interactions have been less frequently studied. The goal of my dissertation is to develop a mechanistic theoretical framework for elucidating the structure and dynamics of below-ground and above-ground mutualistic interactions.Chapter 1 presents a mechanistic model for a pairwise plant-mycorrhizal interaction that combines the facilitative and consumptive aspects of the interaction with explicit nutrient dynamics. By parameterizing the model with empirical data, I find that mycorrhizal fungi act as antagonists rather than mutualists when the fungus’ carbohydrate consumption outweighs the benefit it provides, and that the shift from mutualistic to the consumer-resource realm is governed by soil nutrient availability. The model yields testable predictions in terms of plant and fungal biomasses that can easily be measured in the field. Chapter 2 presents a theoretical framework to investigate the de novo assembly of plant-mycorrhizal communities. Testing the model using empirically measured parameters shows that nestedness, long considered a signature of facilitation, arises only when the constituent specialist-generalist interactions are mostly antagonistic rather than facilitative. Generalist-generalist interactions, in contrast, are more often facilitative because the consumptive effects of fungi are distributed across multiple plant hosts. Chapter 3 presents a mechanistic model of a pairwise plant-pollinator interaction that parallels Chapter 1 in combining the facilitative and consumptive aspects of the interaction with explicit nutrient dynamics. Unlike the fungi studied in Chapter 1, pollinators tend to act as mutualists rather than antagonists. This is likely because pollinators engage in ephemeral interactions with plants compared to mycorrhizal fungi that form permanent associations through hyphae that extend into plant roots or form a sheath outside it. Taken together, these chapters provide novel insights into the mechanisms underlying the structure and dynamics of below-ground and above-ground mutualistic interactions and generate testable predictions framed in terms of easily measurable parameters that can guide future studies and applications.