The science of ethology is concerned with the way external stimuli and internal events cause animals to fight in a particular way. The classification of dog breeds with respect to their relative danger to humans makes no sense, as both, the complex antecedent conditions in which aggressive behaviour occurs, and its ramifying consequences in the individual dog's ecological and social environment, are not considered. From a biological point of view, environmental and learning effects are always superimposed upon genetic influences. Based on the recent developments in the study of ethology, aggression of wolves (Canis lupus L.) and domesticated dogs (Canis lupus f. familiaris) was put into context with respect to other aspects of the lifestyle of wild and domestic canids. Aggressive behaviour does not occur in a biological vacuum. This is also true for domestic dogs and their relationship to human partners. Individual dogs can become highly aggressive and dangerous. Their development and social situation will be presented and discussed in case studies. Finally, there is the question about defining "normal aggression" versus symptoms for maladaptive aggression resp. danger to humans as conspecifics. It is possible to protect the safety of the public and at the the same time practise animal care. Effective animal control legislation must focus on responsible ownership and socialisation of pups f.e. Problems are not unique to some breeds.