Pup mortality is reported to be a significant problem in the dog. The purpose of this thesis was to identify the extent and causes of the mortality and the risk factors. Mortality was classified according to the clinical condition of the pup at birth and the pathological investigation was designed to investigate the validity of this classification. Total pup mortality, excluding elective euthanasia for show reasons, was 18.5%. Perinatal mortality, that is, stillbirths and deaths that occurred in the first week, accounted for 90.9% of these losses. Each breed surveyed exhibited a specific mortality pattern and the results of one breed could not be used to anticipate the outcome in another breed. As a consequence of this, there was a marked difference in the predictor variables, or risk factors, identified for each breed. Birth weight and inter-pup whelping intervals were the most consistent variables that increased the odds of a pup dying. The principal cause of pup mortality was attributed to foetal asphyxia, that is, apparently normal pups subjected to excessive hypoxia during the birth process and they were either still born or born in a distressed condition and subsequently died. Death attributed to foetal asphyxia accounted for 7.8% of all pups born and 42.5% of the total mortality. The majority of these pups (82.2%) died during whelping or in the first 24 hours after birth. The death of just over half of these pups could be directly attributed to dystocia. The remaining pups were compromised during what appeared to be a normal whelping. Neonatal atelectasis, pulmonary congestion, inhalation of amniotic fluid and meconium, leptomeningeal and generalised systemic congestion were the principal pathological findings in these pups. Average birth weights, inter-pup whelping intervals, parity, pup presentation and litter position were all significant predictors of mortality due to foetal asphyxia. The abnormal pup was defined as a pup at birth that was mummified, had died prior to birth, was small for date or had gross congenital defects present. These accounted for the death of 4.9% of all pups born and 26.3% of all losses. The only significant predictors of mortality due to the birth of an abnormal pup were the inter-pup interval and birth weight. Since the abnormality occurred in utero and was not related to the birth process this result had no bearing on the outcome. The death of live born, apparently normal pups, in the neonatal period accounted for 5.7% of all pups born and 31.2% of the total mortality. Over half these losses were attributed to fading puppy syndrome. The remainder were due to mismothering / mismanagement and other miscellaneous causes. The majority of fading pups examined were not normal at birth. Growth retardation and the consequent increased susceptibility to foetal hypoxia, lung pathology indicative of foetal asphyxia and intrauterine and/or very early neonatal infections were the principal causes of mortality attributed to fading puppy syndrome identified in this study. The canine perinate is totally dependent on the bitch both in the uterus and in the immediate post partum period. The investigation of pup mortality can not be divorced from the assessment of maternal health, the influence of the whelping process and the post whelping care of the immature pups by the bitch. These factors must be correlated with gross and histological changes identified in dead pups to determine the sequence of events that contributed to the death of the whelp.