This article describes the development of number concepts between infancy and early childhood. It is based on a diary study that tracked number word use in a child from 12 to 38 months of age. Number words appeared early in the child's vocabulary, but accurate reference to specific numerosities evolved gradually over the entire 27-month period. Reliable performance on standardized number and counting tasks lagged behind accurate number use in informal settings by many months. Although there was evidence that this child labeled mentally represented sets, these instances comprised a very small proportion of the total sample and were usually references to anticipated sets rather than remembered sets.