The recent discovery of the importance of kisspeptin-GPR54 signaling to the onset of puberty in humans has generated intense interest in understanding the mechanisms and pathways of kisspeptin-GPR54 signaling at the gonadotropin-releasing hormone (GnRH) neurons. Using transgenic mouse models, electrophysiology and immunocytochemistry we have shown that kisspeptin strongly activates adult GnRH neurons and that these cells receive a direct input from kisspeptin neurons located in the rostral periventricular region of the third ventricle (RP3V). With respect to the initiation of puberty, data indicate that the GnRH neurons become activated by kisspeptin through a two step process; first, the expression of kisspeptin in RP3V neurons increases abruptly just prior to puberty and, second, the response of GnRH neurons to kisspeptin becomes more pronounced. Recent studies examining the mechanism underlying the up-regulation of kisspeptin expression in the RP3V at puberty, suggest a critical role for circulating estrogens in the week prior to puberty onset. Ovariectomy in the second week of life prevents the up-regulation of kisspeptin expression and this is corrected by estrogen replacement. As GnRH neuron activation is required to initiate estrogen synthesis, these results suggest that the early activation of the GnRH neurons occurs independent of kisspeptin. However, once initiated, circulating estrogens activate RP3V kisspeptin neurons that, in turn, strongly excite the GnRH neurons. Intriguingly, data suggest a similar scenario in the adult whereby estrogen activates RP3V kisspeptin neurons to initiate the GnRH surge leading to ovulation. Thus, it seems likely that an important estrogen-kisspeptin-GnRH neuron signalling pathway develops around the time of puberty and that this is used for the activation of GnRH neurons both at puberty onset and ovulation.