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Broilers are chickens kept commercially under intensive husbandry conditions for poultry meat production. They grow to a slaughterweight of approximately 2.2 kg in 6 weeks. Broilers show a pronounced decrease in behavioural activity during their short life. The aim of this thesis was to gain more insight into the influence of both motivation and ability on behavioural activity in broilers. The distinction between motivation and ability is relevant for the interpretation of behavioural activity in broilers in terms of welfare. Broilers were observed when kept in pens with good facilities and in several ex situ tests where they had to work for food. Broilers showed all normal juvenile chicken behaviours, but the time spenton each behaviourhas changed. Many behavioural patterns were shown in a sitting posture instead of in a standing posture. Performance in short physical tasks depended on motivation and physical ability. Hunger and satiety mechanisms have changed compared to layer chickens, but the motivation to work for food still can be manipulated in broilers. Broilers are willing to peck a key many times or to walk long distances to obtain food rewards, but their performance seemed to be dependent on body weight. Apart from having an abnormal high motivation to eat, broilers are normal biologically functioning animals. Parallel to the effect of overweight on the quality of life in humans, the abnormal high body weight at a young age has a negative impact on the welfare of broilers. Broilers are restricted to display behaviours for which they are motivated imposed by their high motivation to eat, which is associated with their fast growth and high body weight at young age, or by environmental conditions in which they are usually kept.