Despite the importance of potato tubers as a source of food and a means of propagation, the initiation, growth and development of tubers and the factors affecting these processes are not well understood. There are many reported studies of the effects of various factors on the initiation of tubers but, for a number of reasons, few have sought to understand the initiation of tubers under field conditions. Most reports are concerned with experiments carried out in growth-rooms, which often used very short stem sections as planting material and in which environmental conditions, especially light intensity, differed greatly from those normally prevailing during the period of initiation in the field. In almost all field experiments, the timing of initiation was not defined or properly assessed; number of tubers was recorded infrequently and maximum number of tubers was rarely, if ever, established. Despite these deficiencies, the initiation of tubers is still widely regarded as a key developmental stage in the crop's life, having profound implications for subsequent growth and development (Ewing 1990; Ewing & Struik 1992). This view was first expressed 30 years ago (Ivins & Bremner 1965; Bremner & Radley 1966) and its general acceptance has greatly influenced studies of the growth and development of the crop. During the intervening years, the study of tuber initiation has been an important aspect of research programmes at the University College of Wales (UCW) Field Station, Tenby, Pembrokeshire (1972–84) and Cambridge University Farm (CUF), Cambridge (1982–95), which have now produced sufficient data for a re-evaluation of the significance of tuber initiation in field-grown potato crops.