The objectives of the workshop were to review the relevant literature and to address in detail: (1) In vivo percutaneous penetration/absorption methodology; (2) The characteristics of dosage forms designed for application to the skin; (3) Critical factors controlling in vivo drug transport into and across the skin: (4) The use of models in the assessment and evaluation of in vivo percutaneous penetration/absorption; and (5) Bioavailability/bioequivalencc considerations for topical drug products. Scientific knowledge and technology are rapidly evolving in the topical and transdermal drug products area. This report focuses on the methodologies available for the measurement of percutaneous penetration in vivo; each scientific approach is discussed briefly followed by advantages and disadvantages of the methodology.