Aggressive behaviour in dogs is an increasing problem in The Netherlands. In an attempt to find a solution to this problem the Dutch Ministry of Agriculture, Conservation and Fisheries has financially supported a study aimed at developing an aggression test for dogs. The primary goal is to use the test as an instrument for excluding very aggressive individuals of certain breeds from breeding. On the basis of two pilot studies a test has been developed with 43 sub-tests in which a variety of stimuli are presented relating to contexts that are known to elicit aggression in dogs. In the final test, 112 dogs, 75 of which were potentially aggressive breeds (PAB) and a group of 37 “control dogs”, were tested. Questionnaires were used to collect information about the aggressive history of the dog. The results show clear differences in the aggression-eliciting properties of the sub-tests. Dogs with and without biting history differ significantly in their biting/attack behaviour during the test (Mann-Whitney U-test, P = 0.02). This difference is also found for only the PAB-dogs (MWU-test, P = 0.007). For reliability of analysis, 37 dogs were re-tested. The comparison between test and re-test shows a significant correlation for total attack (SPCC = 0.78) and biting/attack (SPCC = 0.68). So that the test can be implemented in practice, two “Models for Unacceptable Aggression (MUAs)” are discussed. To validate the results of the test and the application of the MUAs the results are compared with the biting history of the dogs. The results of an MUA based exclusively on the biting/attack behaviour shows a significant relation with the biting history for all dogs and for the PAB-dogs. On the basis of these results we consider the test to be a useful instrument for the assessment of aggressive tendencies in dogs, provided the test is performed by trained researchers or trained judges and test assistants.