A theory of beef production is presented as a logical basis for discussion. It offers data to support its validity through successful application to commercial beef breeding herds. The five time-related factors which have the greatest effect on herd performance and profitability are considered to be a) the number of heifers in oestrus on the first day of mating which is related to nutrition, body weight, management and previous herd performance; b) the length of the interval from calving to first heat and conception which is related to the time of calving and mating, nutrition and management; c) the gestation length, e.g. Angus 273–279(6) and Charolais 284–96(1)(2)(7); d) the percentage calving in the first 21 days and e) the repeatability of herd performance within limited mating systems over a 3–5 year period.