Age at puberty is a major determinant of lifetime reproductive efficiency of beef cows. Research conducted during the past 20 yr has documented the major endocrine events leading to first ovulation in heifers. The critical event seems to be a prepubertal increase in pulsatile LH secretion. Environment influences timing of puberty onset in beef heifers. Nutrition and season are two of the better-defined variables that have been studied. Age at puberty is related inversely to plane of nutrition. The effect of nutrition on sexual maturation involves effects on timing of the prepubertal increase in LH secretion and seems to involve the LH pulse generating system located in the hypothalamus. The precise mechanism by which nutrition influences pulsatile LH secretion has not been elucidated, but signals reflecting metabolic status seem to be involved. Seasonal conditions of the early (birth to 6 mo of age) and late (6 to 12 mo of age) postnatal periods also influence timing of puberty onset in the heifer. Autumn-born heifers attain puberty at younger ages than do spring-born heifers, and exposure to spring-summer temperatures and photoperiods during the second 6 mo of life reduces age at puberty regardless of season of birth. Photoperiod may be the major seasonal cue that influences puberty onset in cattle. Limited evidence suggests that melatonin, a pineal hormone, is involved with transducing photic stimuli into neuroendocrine signals that influence LH secretion. If the physiological mechanisms mediating the effects of nutrition and season on timing of puberty onset are determined, then management strategies for reducing age at puberty can be enhanced.