Many reviews and descriptions of single-cell protein (SCP) have been published. 14 The name SCP was first used by workers at MIT in 1968. It means in almost all cases the production of a food or feedstuff from single celled organisms (yeast or bacteria) but has been more loosely applied to fungal products. This chapter is not another review of biochemistry, genetics, or microbiology but an attempt to place the results of research effort in these disciplines in the context of the development of the process. In the case of the ICI process begun in 1969 much of the process development work occurred simultaneously with the development of the scientific background; this was one of the reasons for the excitement of working on such a process.