Fifty-one owner-dog pairs were observed in a modified version of M. D. S. Ainsworth's (1969) Strange Situation Test. The results demonstrate that adult dogs (Canis familiaris) show patterns of attachment behavior toward the owner. Although there was considerable variability in dogs' attachment behavior to humans, the authors did not find any effect of gender, age, living conditions, or breed on most of the behavioral variables. The human-dog relationship was described by means of a factor analysis in a 3-dimensional factor space: Anxiety, Acceptance, and Attachment. A cluster analysis revealed 5 substantially different classes of dogs, and dogs could be categorized along the secure-insecure attached dimensions of Ainsworth's original test. A dog's relationship to humans is analogous to child-parent and chimpanzee-human attachment behavior because the observed behavioral phenomena and the classification are similar to those described in mother-infant interactions.