Many drugs act via the receptors and other proteins that mediate cellular signalling. Such proteins can be grouped into several families on the basis of their structural and functional similarities. Examples from each family are examined at the molecular level from a pharmacological, biochemical and biophysical point of view for insight into their structure, their mechanism of action, their modulation by drugs and the underlying dysfunctions toward which the drugs are directed. Basic principles of molecular pharmacology are introduced as a tool for decoding the relationship between dose and response across all families, with an emphasis on the explicit nature of concepts such as potency and efficacy.