Puberty, the transition from childhood to adult body size and sexual maturity, is a complex multi-staged process involving growth acceleration, weight gain and the appearance of secondary sexual physical features over a 2- to 3-year period. Recent genome-wide association (GWA) studies for age at menarche, the onset of the first menstrual period in girls, have identified the first common genetic variants to be robustly associated with pubertal timing. The findings indicate a novel link between microRNA pre-processing and the timing of whole organism growth and development. Our studies also demonstrate that the use of easily measured phenotypic markers, such as age at menarche in girls, in GWA studies can uncover genetic variants with wider relevance to more complex phenotypes, such as pubertal onset and the tempo of growth in both sexes.