This chapter argues that childhood data can be included in longitudinal studies if researchers are cognizant of developmental patterns for vocal tract morphology and the subsequent influence these patterns have on acoustic correlates. It focuses on to familiarize the longitudinal researcher with trends associated with developing vocal tracts, approaches towards accommodating child vowel data in longitudinal studies, as well as potential pitfalls of working with very young children. The chapter describes typical paths of development observed in cross-sectional/apparent time data in which different age cohorts are compared as a proxy for change over time. The chapter analyses the longitudinal corpus to confirm these developmental paths, thus illustrating that changes to the vowel space are predictable across developmental stages. It discusses the normalization procedures can be applied to child data to reduce physiological variation but to maintain sociolinguistic variation. It also discusses the statistical analyses for longitudinal acoustic data.