Ainsworth's 'strange situation' procedure was used to investigate the dog (Canis familiaris) – human relationship. 38 adult dog-owner pairs were observed in an unfamiliar room, intro-duced to a human stranger and subjected to four short episodes of separation. The procedure and behavioural analyses were as similar as possible to those used in studying human infants, except for the inclusion of an extra separation period in which the dogs were left alone in the room with articles of clothing belonging to the owner and stranger. A secure base effect was suggested by the fact that the dogs accepted to play with the stranger more in the pres-ence of their owner than during his or her absence. They also explored more in the presence of their owner, but this appeared to be due to diminishing curiosity over time rather than a secure base effect. The dogs also exhibited a range of attachment behaviours, i.e. search and proximity seeking behaviours when separated from their owner, including following, scratch-ing and jumping up on the door, remaining oriented to the door or the owner's empty chair and vocalising. They also greeted their owner more enthusiastically and for longer durations compared to the stranger. Finally, they contacted the owner's clothing more often and for 3) Corresponding authors address: Prof. 4) This research was supported by funds from Università di Milano to Emanuela Prato-Previde. We are grateful to Marcello Cesa-Bianchi and Marco Poli for allowing us to carry out the work in the Psychology Institute of Università di Milano. We thank Barbara Rotta for her invaluable help in data collection and scoring, Clara Palestrini for helping in running the experiment, Marco Colombetti for reading and commenting on the preliminary draft of the paper. Finally, a special thank to Tipota, a female mongrel, for being our rst pilot subject and to all the owners and dogs that participated as volunteers.