Despite the wide medicinal use of Natural Products (NPs), information about their oral bioavailability remains scant. It is, however, essential to know if the biologically active NPs have a chance to reach their therapeutic targets in vivo. The present thesis aimed at using the Parallel Artificial Membrane Permeability Assay (PAMPA) for an in vitro evaluation of the passive intestinal absorption of NPs, directly in crude plant extracts. Thanks to powerful cheminformatic tools, the main physicochemical properties influencing the passive permeability of representative NPs classes were highlighted, and a free software (Qabsorb) was developed for the in silico prediction of passive intestinal absorption of diverse NPs. A novel PAMPA method was also developed to predict the NPs binding with human serum albumin. This thesis gives a promising and practical approach for a rapid evaluation of oral absorption of NPs, that can be applied to pure compounds and complex mixtures from plants.