The major flux in potato tuber carbon metabolism is the conversion of sucrose through hexose phosphates to starch. The enzymes that mediate this pathway are well characterized, the genes that encode them have been cloned and transgenic plants have been generated. These transgenic studies have confirmed hypotheses based on more indirect methods, but they have also generated new challenges by highlighting the enormous flexibility and complexity inherent in plant metabolism. The investigation of the sucrose-to-starch transition in potato tubers is an excellent example of how the discipline of molecular plant physiology is evolving at both the scientific and technical levels.