The mechanisms that control the onset of puberty remain within the purview of the neurobiologists who first recognised the concept of removal of a restraint factor to permit reawakening of puberty. The discovery of a ligand-activated G protein receptor-signalling pathway upstream of the GnRH pulse generator adds further weight to the role of the hypothalamus in the central regulation of puberty. The physical pointers to puberty are the mainstay of assessing timing and tempo in a clinical setting but non-invasive and indirect methods of assessment only are appropriate for population studies in normal children. That puberty can be regarded as a sensor recognises the observation of secular changes in pubertal timing and perhaps qualitative aspects of tempo. The influences on the sensor include marked changes in nutrition and an environmental exposure to low-dose chemical mixtures interacting with a polygenic background.