Grafting is an ancient agricultural practice that joins the root system (rootstock) of one plant to the shoot (scion) of another. It is most commonly employed in woody perennial crops to indirectly manipulate scion phenotype. While recent research has focused on scions, here we investigate rootstocks, the lesser-known half of the perennial crop equation. We review natural grafting, grafting in agriculture, rootstock diversity and domestication, and developing areas of rootstock research, including molecular interactions and rootstock microbiomes. With growing interest in perennial crops as valuable components of sustainable agriculture, rootstocks provide one mechanism by which to improve and expand woody perennial cultivation in a range of environmental conditions.