An account is given of the techniques and problems associated with census work in rookeries of the Emperor penguin (Aptenodytes forsteri) during its breeding season in the Antarctic winter. Results for population and breeding success from four rookeries over a seven-year period are presented and are compared with published data from four other rookeries. The available evidence suggests that Emperor penguin rookery populations, although capable of rapid increases, generally remain stable; it does not suggest that the species is threatened with imminent extinction. The world population of Emperor penguins is estimated to be at least 120,000 pairs breeding in at least twenty-one rookeries. Factors controlling rookery population are considered, with particular reference to breeding failure, the incidence of which is found to be generally high and subject to wide variation between rookeries and seasons. Causes of breeding failure are briefly discussed, with emphasis upon the influence of the rookery site. An observed relation between the adult rookery population during the creche stage and that during inoubation suggests a method of roughly estimating adult population and breeding success from a single late-season count. Reassessment of published data for the Cape Crozier rookery suggests that breeding behaviour and chick development at this rookery may be atypical, and that the incidence of breeding failure may have been even more severe in the past than previous authors have estimated; a consideration of the probable causes suggests a reason for the Emperor penguin's preference for stable rookery sites.