Improving the nutritional quality of maize is necessary to meet the growing demands of maize consumption. Maize could be the main source of protein. Several parameters, however, limit the supply and quality of corn protein. This chapter reviews the mechanisms that determine these limitations, including transcriptional and post-transcriptional regulation of gene expression, compartmentalization of proteins during seed development. It also describes mutants in maize that interfere with these mechanisms and how genetic approaches could be taken to improve protein quality traits. It is important to realize that sulphur and nitrogen storage in maize seeds have to be addressed differently because of their source-sink relationship. This chapter deals with all these aspects.