Permeability is the velocity of passage of a drug through a biological lipid membrane. There are multiple mechanisms of permeability: passive transcellular diffusion, paracellular, efflux transport, uptake transport, and endocytosis. Permeability is appreciably involved in processes that affect in vitro biology, in vivo efficacy, pharmacokinetics, and toxicity, so it is of great interest during drug discovery and development. Passive transcellular diffusion is the predominant mechanism for intestinal absorption of drugs. It increases with lipophilicity and decreases with hydrogen bonding, polarity, and molecular size. Transporters increase or decrease the net permeability by uptake or efflux, respectively. Permeability can be improved by modifying structures to add lipophilicity, remove hydrogen bonding, reduce polarity, reduce molecular weight, or change transporter affinity.