This chapter provides a comprehensive introduction to essential pharmacological principles relevant to anaesthesia and critical care practice. Beginning with an overview of drug action mechanisms, the chapter discusses receptor interactions, enzyme modulation, ion channel blockade, and transport system interference. Core pharmacodynamic concepts such as agonism, antagonism, potency, and therapeutic index are clearly defined, providing a solid foundation for clinical application. The importance of drug interactions—classified as physicochemical, pharmacokinetic, and pharmacodynamic—is examined, highlighting practical implications for anaesthesiology. The chapter also offers a detailed exploration of clinically significant receptor classes, including G protein-coupled receptors (GPCRs), ligand-gated ion channels, and nuclear receptors. Enzyme induction and inhibition, particularly the role of cytochrome P450 enzymes, are discussed, emphasizing their impact on drug metabolism and patient safety. The concepts of first-order and zero-order kinetics are explained, underscoring their significance for dosing accuracy and toxicity risk management. Finally, the chapter addresses isomerism, illustrating how structural and stereochemical differences influence drug potency, efficacy, and safety. By bridging fundamental concepts with clinical relevance, this chapter equips anaesthesia trainees with essential knowledge to optimize therapeutic outcomes and anticipate pharmacological challenges.