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SUMMARY .1. Observations were made on wild and captive Jays, chiefly from June 1946 until October 1950, in and near London. 2. An account is given of the various posturings and call‐notes and the circumstances under which they are used. Vocal mimicry and “song” are discussed and the copied sounds used by a captive pair are listed, together with the stimuli eliciting them and their emotional implications. 3. Some aspects of the breeding cycle are described and discussed, particularly with reference to the spring gatherings, the behaviour of the nestlings and parental care. It is suggested that the primary function of the spring gatherings is to facilitate the meeting of single birds ready to pair. 4. Reactions to enemies are discussed, together with the behaviour of captive birds towards stuffed owls and hawks and a live Tawny Owl. Innate recognition of owls as enemies seems to be weak and subject to individual variation. Black corvids appear to be instinctively recognized as potential nest‐enemies and are liable to be attacked on coming into the nest area. 4. Dominance and intra‐specific aggressive behaviour are described. Among captive birds males normally dominate females, but the female of a pair that have bred for three years running is regularly dominant over the male while she is incubating or brooding young. 5. Feeding behaviour and feeding techniques are noted. Acorns probably form the largest single item in the diet of adult birds. Much time is spent burying acorns, and at least on some occasions the hiding place is remembered. The reactions of captive birds to various warningly coloured insects are described. 6. The behaviour of two incubating female Jays points strongly to the conclusion that where Jays are abundant the high percentage of nesting losses may be due to their robbing nests of their own species. 7. The anting of captive Jays is described. Unlike other birds, they do not, when anting, appear to hold ants in the bill or to place them with the bill amongst the plumage. 8. The play of Jays, like that of ducks and pigeons, consists of “emotion‐dissociated fleeing reactions”, which suggest an accipitrine hawk, probably the Goshawk, as a major predator in natural conditions.