Direct interactions among conspecific and heterospecific animals are often mediated by aggressive behavior. We analyze the ecology and evolution of resourc-related heterospecific aggression (HA) by reviewing and meta-analysing 126 studies, contrasting HA with conspecfic aggression (CA), and discussing terminological confusions and conceptual models. HA occurred in 78% of tests (n = 459), suggesting a high prevalence and potential effect on niche use and community structure. The benefits of both CA and HA are linked to resource defensibility and abundance, yet HA can change independently of CA. Ecological inferences about HA are often weak because they assume that interference always results from resource competition, and evolutionary inferences made by comparing HA to CA are also weak because they usually ignore history. We believe that comparisons between situations where a focal species is allopatric from and sympatric with a heterospecfic competitor provide better opportunities to test hypotheses about HA. In general, according to our data set, aggression was higher with increased resource overlap as expected, both because CA was greater than HA, and HA was greater within compared to between genera. Progress in understanding HA requires distinguishing traits (aggressive behavior) from interactions (agonism, interference), as well as from the ecological and evolutionary causer (competition, ancestry) and consequences (dominance, territoriality, exclusion) of those interactions.