Indomethacin (IMC)-and ibuprofen (IBP)-loaded polystyrene microparticles were prepared by emulsion- solvent evaporation method from an aqueous system. Comparing the in vitro drug release profiles obtained from microparticles and marketed SR products allow the selection of an optimized formulation for IMC and IBP. Bioavailability studies in human volunteers indicate that while no differences among the pharmacokinetic parameters were evidenced between the optimized and SR- formulations of IMC, all parameters except ka and ke of the optimized IBP formulation were lower than that those of the SR-formulation. Although statistically significant in vitro: in vivo correlations were observed individually for the optimized and for the SR- formulations, values of retard quotient (R?), revealed an almost same and a lower retardation for the optimized IMC and IBP formulations, respectively. With this coverage on in vitro and in vivo evaluation of microparticles, this book is an essential reference for researchers in pharmaceutical industry who develop oral microparticulate dosage forms.