The main part of this thesis sets out to study the sequential organisation of sucking during normal breast feeding. Data on breast feeding are scarce when compared with the large body of work on sucking during bottle feeding. One of the initial outcomes is that on the first breast, as compared with the second, sucking is slower overall. The widely-quoted dichotomy between nutritive and non-nutritive modes of sucking is not upheld. Rather, the organisation of sucking exhibits complete gradation between these two rhythms, and pausing is always a feature of sucking on the breast. Sucking patterns were also seen to change progressively through the feed, and attempts are made to account for these changes using several different approaches. Sucking patterns, in particular sucking rate, during breast feeding appear to be determined largely by the rate of milk flow rather than by the baby's hunger, fatigue or behavioural state, although the concurrent changes in milk composition cannot be ruled out since changes in milk flow and composition on the breast cannot be dissociated. However, the probability of the baby sucking at any one time is less strongly correlated with milk flow rate. An attempt is made to place the findings in their evolutionary and social context.