This chapter contextualizes the dog-human relationship in the dog's origin as a scavenger on the fringes of human settlements over 15,000 years ago. It then reviews the evidence for unique evolved cognitive structures in dogs that could explain their success in a human-dominated world. Failing to find evidence of unique human-like social-cognitive capacities I then review uncontroversial facts of dogs' basic behavioral biology, including reproductive and foraging behavior and, particularly, affiliative and attachment-related behaviors. This leads to consideration of dogs' social behavior, both conspecific and toward other species, especially humans. I draw attention to a seldom-noted apparent contradiction between dogs' stronger affectional bonds toward humans than toward members of their own species. Dogs' social groups also show steeper social hierarchies accompanied by more behaviors indicating formal dominance than do other canid species including wolves. I resolve this contradiction by proposing that dogs' intense sensitivity to social hierarchy contributes to their willingness to accept human leadership. People commonly control resources that dogs need and also unknowingly express behaviors which dogs perceive as formal signs of dominance. This may be what Darwin was referring to when he endorsed the idea that a dog looks on his master as on a god. Whatever the merits of this idea, if it serves to redirect behavioral research on dogs in human society more toward the social interactions of these species in their diverse forms of symbiosis it will have served a useful function.