An important event in the life cycle of the potato is the initiation of new tubers. This occurs when underground stolons swell radially near the tip and stolon elongation growth stops. The timing of tuber initiation depends strongly on environmental conditions including length of the photoperiod and temperature. Tuber formation is promoted by short days and cool temperatures and inhibited by long days and high temperatures. Plant nutrient status, as indicated by C:N ratio, and hormones, especially gibberellins, also contribute to the complex regulatory network that controls tuberization. Molecular players linking plant genetics and environmental conditions to tuber initiation and enlargement have been described recently. These include StCDF1, which indirectly promotes tuberization and has a strong effect on maturity. The phloem, a component of the plant vasculature, plays a critical role as a conduit for carbohydrate, mRNA and protein signals that coordinate below-ground tuber development with above-ground vine development and perception of the environment. Phloem-mobile signaling molecules that promote tuber formation include StSP6A protein and StBEL5 mRNA. StSP6A promotes tuber formation in stolons as part of a multi-protein complex that has functional and structural similarities with the complex that promotes flowering. Production of StSP6A is inhibited by elevated temperatures. This finding illustrates how observations made in the field are being linked to the molecular model for tuber initiation.