This project addresses two key questions in population history: how were cities transformed from demographic sinks into self-sustaining populations; and when and why did class differences in mortality emerge? The project uses a novel source of evidence, the baptism fee books of St. Martin-in-the-Fields, to generate improved estimates of infant mortality in London c.1750-1825, and the first estimates of infant mortality rates by social status for this period. The dataset will reveal whether class differences emerged over the period 1750-1825, and the extent to which this coincided with a decline in smallpox mortality, or with the development of other differences, for instance changes in breastfeeding patterns, or in survival rates in the first weeks and months of life, when maternal health and obstetric practices are the main determinants of mortality. Critically, the richness of the St. Martin-in-the-Fields datasets not only provides the key types of information required to establish the extent and causes of social class differences in mortality, but also makes it possible to correct for a number of potential biases that could distort these measurements.