Pharmacology is the study of the actions and effects of drugs on living organisms. There are two main branches of pharmacology: pharmacokinetics (what the body does to the drug) and pharmacodynamics (what the drug does to the body). In this chapter, the authors define useful concepts from both areas first, then the pharmacokinetics of essential oils followed by pharmacodynamic interactions of essential oil constituents with relevance to disease states of various body organs and tissues. Pharmacodynamics is concerned with the following: how drugs bind to drug targets; biochemical, physiological and possible adverse effects of drug binding; potency, specificity and efficacy of drugs; drug interactions with food, herbs and other drugs. Binding affinity is the extent to which a molecule binds with another molecule. The biomedical model of pharmaceutical medicine has as a central myth the ‘lock and key’ concept of drug action, that is, a drug (key) activates or blocks a single target (lock).