In the first part of this thesis, the behavioural development of domestic piglets kept in a socially and ecologically rich outdoor environment, the Pig Park, is traced. In the second part, the effects on behaviour of artificially rearing piglets in individual incubators for their first seventeen days are examined. By con?trast with the Pig Park, the incubators provided a very barren and restricted environment. Focal animal and scan sampling techniques were used to collect data, which was then subjected to frequency, sequence and cluster analyses. In the Pig Park, piglets remained in and around their farrowing nest during their first week, after which they started to rest in other nests and to interact with other pigs. After the formation of teat preferences, they were mainly responsible for locating and defending their teat from other piglets. Two piglets switched mothers when they were two and six weeks old respectively, while others sometimes suckled opportunistically from vacant teats other than their own. Piglets were aware of the activities of those around them and social facilitation occurred, especially between littermates. Weak dominance relationships were demonstrated between pairs of piglets, but the outcome of an interaction also seemed to be affected by the context and the motivation of the par?ticipants. Social interactions and spatial associations were more likely to occur with the dam or a piglet than with a juvenile or other adult. Within litters, strong and lasting preferences for particular individuals were not found. Locomotory play was the most common form of play, especially between three and six weeks ofABSTRACT age. The observations are discussed with reference to the selection pressures acting on wild piglets and the degree to which behaviour has been modified through domestication. In the incubators, high levels of high-pitched vocalization were attributed to frustration induced by an unpredictable supply of milk. Stereotyped rooting and biting, and massaging of the nipple drinker were also observed. When transferred to flat-deck cages and placed in pairs, the piglets showed abnormally high levels of many social behaviour patterns, including massaging, sucking, levering and circling. With time, their behaviour became similar to that of control piglets reared by sows, with the exception that only incubator-reared piglets massaged their nipple drinker. The results are related to the welfare of piglets and the effects of environment on motivation and behaviour.