Jeffrey K. Aronson and Robin E. Ferner, both practitioners of clinical pharmacology, bring the whole discussion of dosage up to date. Their chapter reflects their own present-day research on reactions to medicines and thus on questions of dosage. They draw our attention to the Law of Mass Action propounded by two Norwegian scientists in 1864, which was the foundation of quantitative treatment of pharmacological phenomena, and which ‘dictates that the magnitude of a pharmacological reaction increases with increasing dose’. Aronson and Ferner trace much subsequent experimental researches subsequent to the establishment of that Law, including the introduction of the concept of receptors, and concentration-effect and dose-response lines of inquiry, bringing the whole discussion up to the present day. It seems that it does indeed all depend on the dose, though many other factors are involved.