To be economically feasible, production of most secondary metabolites by plant-cell culture requires high cell densities. In standard reactor configurations such as stirred-tank and airlift vessels, large-scale culture of plant cells encounters problems of shear damage, poor mixing, and inadequate mass transfer of oxygen. Immobilised cells and organised tissue such as embryos, roots and shoots often give enhanced levels of production; however, design of appropriate reactors is at an early stage. This paper identifies the rate-limiting processes in plant-cell culture, and discusses ways for improving reactor performance.