Salmonid life-history patterns vary within and between species, and within and between populations. Two major developmental conversions (sensu Smith-Gill) occur in the life of salmonids: smolting, in which freshwater adaptations are exchanged for marine ones; and sexual maturation. Each process is circannual, endogenously rhythmic but synchronized by photoperiod. Each involves a choice of developmental route, whose direction depends on the individual's responses to prior feeding opportunity, and its current metabolic performance. Data are presented from laboratory and farm experiments on Atlantic salmon, Salmo salar L., designed to test this developmental model. The findings are used to interpret variation in these developmental characteristics, and their consequences for life-history patterns, evident over the geographical range of Atlantic salmon and other salmonids.