This chapter presents the definition of a storage protein as "any protein accumulated in significant quantities in the developing seed which on germination is rapidly hydrolyzed to provide a source of reduced nitrogen for the early stages of seedling growth". Legume storage proteins were first described by Osborne as salt-soluble proteins and were grouped into two classes, vicilins and legumins, distinguishable in size and sugar content. Legumins are unglycosylated globulins with sedimentation coefficients in the range of 11–12 S and have a complex oligomeric structure. Legume storage proteins accumulate in vacuoles. At the end of maturation, cotyledonary cells contain numerous small vacuoles almost completely filled with storage proteins. In pea, conditions of sulfur deficiency negatively affect accumulation of legumin, which is higher in sulfur amino acids, and stimulates the accumulation of vicilin. Nutritional improvement could also be achieved by modification of lectin content or toxicity.