Biologists since Darwin have recognized that domestication, where species are selected to live in human-controlled environments, exerts strong selection on organisms and dramatically impacts their evolutionary trajectories. Across domesticated mammal species, characteristic morphological, physiological and behavioural changes occur simultaneously, as correlated traits, a phenomenon known as the domestication syndrome. Key behavioural alterations are connected with the domestication syndrome, in which domesticated animals express decreased aggression and fearfulness alongside increased sociability and playfulness compared to their wild counterparts. To investigate various aspects of the behavioural implications of domestication, we used the dog (Canis familiaris) and its extant ancestor, the grey wolf (Canis lupus), as our study species. Since we currently lack quantitative confirmation that correlated changes in behaviours follow domestication, we evaluated correlations among sociability, aggression, fearfulness and playfulness in more than 90,000 dogs in Paper I. Contrary to expectations, we found weak support for behavioural correlations in modern dog breeds, but observed exaggerated effect sizes of correlations in ancient breeds. We suggest that while selection on suites of behaviour have been relevant during early dog domestication, a recent shift in selection pressures in modern dog breeds affects the expression of domestication-related behaviours independently. In Paper II we therefore contrasted the expression of sociability, aggression, fearfulness and playfulness during domestication in wolf hybrids and dogs, and found that while wolf hybrids were less playful and overall more fearful than dogs, they were not less social or more aggressive than dogs. Our results suggest that behavioral alterations during domestication do not necessarily occur in concert as predicted by the domestication syndrome and point to an important, but previously overlooked, role of selection on playfulness during the domestication of dogs. Finally, while it has been established that behavioural responses in adult domesticated animals are altered compared to ancestral species, we know little about when such species differences occur. We therefore conducted two studies addressing the effects of domestication on behavioural ontogeny. First, we examined the ontogeny of sociability, playfulness, aggression and fearfulness in wolves and dogs in Paper III and found that while wolves became less social and less playful than dogs at 12 and 16 weeks of age, we found no species differences in the development of fear. Our results suggest that the alteration of behaviours in the domestication syndrome do not develop simultaneously, and that species differences in fear might not occur until later in ontogeny. Then, in Paper IV we present the first extended examination of the development of fear behaviour in wolves and dogs throughout their first 26 weeks of life. We found that while dogs, but not wolves, expressed decreased fear towards novelty with age, this did not result in a species difference in fear response until 26 weeks of age. Our results suggest that differences in fear expression between wolves and dogs occur late during juvenile development and are caused by a loss of sensitivity towards novelty with age in dogs. Together, the four papers in this thesis highlight the need for a re-evaluation of the behaviours hypothesized to be shaped by domestication.