The oral route is most preferred for chronic drug therapy. Poor oral bioavailability has the consequences of more variable and poorly controlled plasma concentrations and drug effects, in addition to possibly increased product cost. In this review, the most common causes of low oral bioavailability are categorized, and formulation strategies to improve bioavailability are summarized. Various methods that can be used to help identify the cause of low bioavailability are discussed. The focus of this article is on poor membrane permeation and presystemic degradation problems; solubility/dissolution rate problems are discussed only briefly. Poor membrane permeation and presystemic degradation problems are typically encountered in the efforts to develop oral proteins, peptides, and peptide mimics. Formulation strategies reviewed include the use of metabolism inhibitors, membrane permeation enhancers, ion pairing and complexation, and particulate carriers. Also reviewed are lipid and surfactant formulations, which have been shown to increase bioavailability by various mechanisms and which are only beginning to be understood and optimized.