The domestic cat is the only member of the Felidae to form social relationships with humans, and also, the only small felid to form intraspecific social groups when free ranging. The latter are matriarchies, and bear only a superficial similarity to those of the lion and cheetah, which evolved separately and in response to very different selection pressures. There is no evidence for intraspecific social behavior in the ancestral species Felis silvestris, and hence, the capacity for group formation almost certainly evolved concurrently with the self-domestication of the cat during the period 10,000 to 5,000 years before present. Social groups of F. catus are characterized by cooperation among related adult females in the raising of kittens from parturition onward and competition between adult males. Unlike more social Carnivora, cats lack ritualized submissive signals, and although "peck-order" hierarchies can be constructed from exchanges of aggressive and defensive behavior, these do not predict reproductive success in females, or priority of access to key resources, and thus do not illuminate the basis of normal cat society. Cohesion in colonies of cats is expressed as, and probably maintained by, allorubbing and allogrooming; transmission of scent signals may also play a largely uninvestigated role. The advantages of group living over the ancestral solitary territorial state have not been quantified adequately but are likely to include defense of permanent food sources and denning sites and protection against predators and possibly infanticide by invading males. These presumably outweigh the disadvantages of communal denning, enhanced transmission of parasites, and diseases. Given the lack of archaeological evidence for cats kept as pets until some 4,000 years before present, intraspecific social behavior was most likely fully evolved before interspecific sociality emerged. Signals directed by cats toward their owners fall into 3 categories: those derived from species-typical actions, such as jumping up, that become signals by association; signals derived from kitten-to-mother communication (kneading, meow); and those derived from intraspecific cohesive signals. Social stress appears widespread among pet cats, stemming from both agonistic relationships within households and territorial disputes with neighborhood cats, but simple solutions seem elusive, most likely because individual cats vary greatly in their reaction to encounters with other cats.