Calving performance records were used to study the inheritance of calving ease and livability. Data were 965,417 calving records from Angus field data and 6,092 records from a crossbreeding experiment designed to produce three synthetic breeds of small, medium and large size. The Angus data were collected between 1972-85 and those of the synthetic cattle between 1978-87. The objectives of the study were: (1) to characterize the environmental and genetic factors which influence calving ease and livability; (2) to estimate variance/covariance components; (3) to determine selection criteria;Two Angus data sets, composed of 19 and 34 herds were used to estimate genetic parameters using a sire-maternal grandsire model. The frequency of dystocia was low in both the Angus (3.61% assisted births) and the synthetic cattle (7.40% assisted births). Sex of calf and parity of dam were important causal components of calving difficulty. In the synthetic cattle, farm and size were also significant causes of dystocia. The Angus data indicated a threshold for birth weight on calving difficulty at around 70 lbs;For calving ease, the genetic correlations between direct and maternal components were estimated at -0.86 and -0.68. Maternal heritability (0.27 and 0.20) was higher than the direct heritability (0.21 and 0.07). There was little genetic variation in livability. Parameter estimates were within the expected range in the sample of 19 herds. Direct heritability was 0.04 while maternal heritability was 0.09 and the direct/maternal genetic correlation was -0.75. In the synthetic cattle, heritabilities for birth weight, calving ease and livability were 0.25, 0.05 and 0.06 respectively. The genetic correlation between birth weight and calving ease was 0.50, while livability and calving ease were essentially uncorrelated. In general, there were major differences among the three size groups. Calving difficulty was higher in the large group and about the same in the small and medium groups. Birth weight tended to increase linearly across the three size groups with the increase from small to medium, to large. Differences among generations were less variable with no specific trends.