Ontogenetic variation is the most fundamental biological aspect of an organism, and stage-structured (size-structured) prey–predator relationships have increasingly been studied for a better understanding of food-web dynamics.  However, little is known about stage-structured mutualism and its ecological consequences.  First, I show that a synthesis of the literature across different mutualisms (e.g. pollination, seed dispersal, nutritional and defensive) demonstrates that mutualism is commonly stage-structured in nature.  In addition, I briefly overview my recent theoretical studies on the ecological consequences and the evolutionary mechanisms of stage-structured mutualism in omnibus form.  In conclusion, I suggest that both empirical and theoretical efforts are needed to collect and incorporate individual-level interaction data into community ecology theories, which will provide valuable insights not only into mutualism-mediated community dynamics but also into biodiversity conservation and ecosystem management.