The affinity of a drug candidate for a biological membrane (its lipophilicity) is closely related to the pharmacologically crucial events of absorption, biodistribution, metabolization and excretion. The evolution of knowledge of biological membranes during the past two decades contrasts with the rudimentary parameter most commonly used to assess lipophilicity: P(o/w), the octanol-water partition coefficient. P(o/w) is especially unrealistic when testing molecules that are polar or partially charged. By contrast, lipid vesicle-based methods determine the extent of the actual partition of a drug to a membrane much more accurately, and have the additional advantage of enabling the choice of the lipid composition considered most suitable to answer a specific biological or pharmaceutical question. In addition, some of these methods are appropriate for high throughput screening, thus shifting determinations of membrane partition to a more preliminary stage of drug development. This streamlines research and development, by saving the time and money that would be spent on unpromising leads.