This chapter outlines the fundamental principles of pharmacokinetics as applied to anaesthesia and critical care. The chapter begins by examining drug absorption and bioavailability, detailing how routes of administration, molecular characteristics, and first-pass metabolism influence systemic drug availability. It further explores hepatic metabolism, clearly explaining Phase I (modification) and Phase II (conjugation) reactions, alongside physiological, genetic, and pathological factors affecting these processes. Key pharmacokinetic parameters—including volume of distribution, clearance, and half-life—are defined, highlighting their interrelationships and clinical implications for precise dosing and management of drug therapy. The chapter introduces compartment models, ranging from simple one-compartment to complex three-compartment models, essential for predicting drug distribution and elimination. Special attention is given to intravenous anaesthetic agents and target-controlled infusions, providing trainees with practical insights into managing pharmacokinetic variability across diverse patient populations. By connecting theoretical concepts to clinical practice, this chapter ensures trainees have the foundational knowledge required to accurately predict drug behaviour, individualize anaesthetic strategies, and enhance patient safety.