This paper summarises current knowledge of the three populations of harbour porpoises in US and Canadian Atlantic waters. During the summer, the populations are centred in the Gulf of Maine/Bay of Fundy region, the Gulf of St. Lawrence and off the east coast of Newfoundland. Winter distributions of all the populations are largely unknown. These populations are thought to be reasonably discrete, as shown by studies of mitochondrial DNA, reproductive schedules, contaminants and radio tagging. From the Gulf of Maine/Bay of Fundy population, the average abundance was 47,200 (95% CI= 39,500 to 70,600) for 1991 and 1992. Based on observer programs, in 1993 the bycatch in the groundfish sink gillnet fishery from the US Gulf of Maine was 1,400 (95% CI= 1,000 to 2,000) and 200 to 400 animals from the Canadian Bay of Fundy. Currently, time-area closures and acoustic devices are being used to reduce the bycatch in the Gulf of Maine and Bay of Fundy. From the Gulf of St. Lawrence, no abundance estimates are available, and based on mail surveys conducted in 1989 to 1991, approximately 2,800 harbour porpoises were caught annually in fishing gear. Reliable abundance and bycatch estimates are not available from the Newfoundland population, although it is thought that more that 1,000 animals annually were caught in the 1980’s. Known life history parameters include: a mean age at sexual maturation for females of about three years; females reach larger asymptotic lengths (155cm) than males (144cm); reproduction is annual; gestation lasts approximately 10.6 months; and lactation lasts 8 to 12 months. Conception probably occurs later for the Newfoundland population than for the Gulf of Maine/Bay of Fundy population. During the summer, harbour porpoises primarily feed on Atlantic herring. The Gulf of Maine/Bay of Fundy population also feeds on silver hake, while the Gulf of St. Lawrence population feeds on capelin and redfish. Organochlorine contaminants were lowest in harbour porpoises from Newfoundland and highest in animals from the Gulf of Maine/Bay of Fundy, though these levels are lower than those measured a decade ago. Mean concentrations of cadmium and zinc contaminants were significantly higher in harbour porpoises from Newfoundland, perhaps reflecting the higher cadmium levels found in many Arctic cetaceans.