Organisms interacting with echinoids are common and produce diverse traces that are often distinctive and can be preserved in the fossil record. Thus, echinoids provide a wealth of information regarding the role of biotic interactions as drivers of ecological and morphological adaptations over macroevolutionary timescales. Studies documenting interactions with echinoids and the resulting traces have become more numerous. This Element reviews the ecologies of skeletal trace-producing interactions on echinoids in Modern ecosystems and the recognition of those biogenic traces in the fossil record. The authors explore diversification and morphological trends in Meso-Cenozoic echinoid clades and associated predator and parasite groups in the context of selective pressures brought about by the evolution of these biotic interactions. Their intent is that this review promotes additional studies documenting the intensity of biotic interactions with echinoids in both Recent and fossil assemblages and highlights their potential to advance our understanding of ecosystem functioning and evolution. This title is also available as Open Access on Cambridge Core.