The authors firstly review the literature dealing with drug absorption sites in the gastrointestinal tract. Descriptions are given of the methods used in determining the location of these sites, and the advantages and disavantages of each method are critically discussed. The results obtained concerning the absorption sites of the drugs used in the in vivo methods studied are given in a tabular form and several factors influencing drug absorption are briefly reported. Mechanisms of drug absorption in the human body and their influence on absorption sites are examined. Finally, there is a discussion of various dosage forms which are used for targetting drug absorption to specific sites.