Research Findings: This study examined age-based developmental trajectories of four key number sense (NS) skills within an accelerated longitudinal design. Using data from 408 preschoolers, ages 45 to 68 months, linear and latent basis growth curves were evaluated. Results indicated growth across NS skills was best represented as nonlinear for all but number naming. Significant variation in initial performance at 45 months was evident across skills, as was significant variation in growth for all skills but number naming. Practice or Policy: Mathematical skills at kindergarten entry have been supported as strong predictors of subsequent academic achievement as well as social emotional skills. Results of this study advance our understanding of the development of key skills in early mathematics during the preschool period and provide additional information about patterns of development of key NS skills across the preschool years. This information can be used to inform measurement development, consider timing of assessment practices, and promote targeted instruction in early education.