This is the first of two installments examining early puberty in girls. The first paper will discuss secular trends in onset of puberty and the possible mechanisms to explain these developments. The potential etiologies examined will include the role of endocrine-disrupting chemicals and obesogens, the impact of body mass index and obesity, genetic and biologic pathways, and the influence of lifestyle behaviors. The second paper of the two-part series will examine the potential health impacts of early puberty on young and adult women and offer suggestions for clinical management and public health prevention.