Although several routes of administration can be considered for new drug entities, the most popular remains the oral route. To predict the in vivo performance of a drug after oral administration from in vivo data, it is essential that the limiting factor to absorption can be modelled in vitro. In the case of BCS class II drugs dissolution is rate-limiting to absorption, so the use of biorelevant dissolution tests can be used to predict differences in bioavailability among different formulations and dosing conditions. To achieve an a priori correlation, the composition, volume and hydrodynamics of the contents in the gastrointestinal lumen following administration of the dosage form must be accurately simulated. Four media have been chosen/developed to model composition of the gastric and intestinal contents before and after meal intake. These are SGF, milk, FASSIF and FeSSIF, which model fasted and fed state conditions in the stomach and small intestine respectively. Using these media, excellent correlations have been obtained with the following poorly soluble drugs: danazol, ketoconazole, atovaquone and troglitazone. In all cases, fed vs. fasted state effects can be predicted from dissolution data and, where several formulations were available for testing, dissolution tests could also be used to determine which would have the best in vivo performance.