The importance to elucidate the mechanisms of drug distribution in tissues has been discussed in this review. By considering several factors such as tissue binding, plasma binding, membrane permeability and tissue interstitial fluid space, the tissue distribution states of drug have been characterized based on the biochemical and pharmacokinetic studies. These physiological pharmacokinetic analyses have been applied to evaluate the determinant factors of inter-organ, drug to drug, interspecies or age related changes of tissue distribution of drug. The mechanisms of membrane permeability of drug which could play the most important role to regulate the drug distribution have been discussed. The drug distribution to the brain is strictly restricted by the function of the blood-brain barrier (BBB). The drug transport mechanisms at the BBB have been discussed. Especially, the role of carrier-mediated and absorptive-mediated transport systems at the BBB has been focused on.