Pediatric drug development can be difficult, with logistical, technical and ethical challenges. The US and EU require products developed for adults to be tested in children with similar conditions, absent a waiver. The majority of pediatric labeling over the past 15 years involved some degree of extrapolation of adult data to children. The promise of more efficient and effective pediatric drug development must be balanced by the peril of approving an ineffective drug by wrongly assuming that prior adult data can be incorporated into the pediatric program. This presentation will examine the assumptions behind the use of extrapolation, and the need to support this assumption both qualitatively (clinically) and quantitatively (statistically). Topics will include: the relevant characteristics on which to establish the similarity of two populations; how these characteristics may differ when using prior adult data to establish pediatric dosing, efficacy, and/or safety; testing the validity of an extrapolation assumption empirically and statistically; conducting adult development to support the use of extrapolation in pediatric drug development; and facilitating multi-disciplinary teams.