Tectonic or subduction erosion refers to the removal of upper-plate material from the forearc at convergent margins. Subduction erosion has been suggested to represent a major process associated with the transfer of crustal material into the Earth’s mantle at subduction zones. However, few studies have attempted to trace the fate of eroded forearc crust beneath volcanic arcs, where the eroded crust might first emerge after mixing with the upper mantle, owing to the formidable challenge associated with quantifying the rate of subduction erosion and the contribution of eroded crust to arc magmas. In this Review, we summarize the evidence for subduction erosion at convergent margins and show that, through integration of geochemical and geological data in arc settings where critical crustal lithologies can be accessed, quantification of the contribution of eroded forearc crust to arc magmas is possible. We further emphasize the importance of establishing arc–forearc compositional links and illustrate the role of arc petrogenetic models for determining whether the eroded forearc crust contributes substantially (that is, greater than a few percent) to the construction of new arc crust in subduction zones or whether it is primarily exported to the deeper mantle.