The assessment of percutaneous permeation of molecules is one of the main steps in the initial design and later in the evaluation of dermal or transdermal drug delivery systems. The literature reports numerous ex vivo, in vitro and in vivo models used to determine drug skin permeation profiles and kinetic parameters, some studies focusing on the correlation of the data obtained using these models with the dermal/transdermal absorption in humans. This paper reviews work from in vitro permeation studies to clinical performance, presenting various experimental models used in dermal/transdermal research, including the use of excised human or animal skin, cultured skin equivalents and animals. Studies focusing on transdermal absorption of a series of drug molecules and various delivery systems as well as mathematical models for skin absorption are reviewed.