This book deals with measurements and understanding, evaluating, and reporting uncertainties. Its goal is to present fundamental concepts rather than delve into professional expertise. It does not include formal axiomatisation, rigorous proofs, or demonstrations, typically found in specialised textbooks. It is not a monograph; it does not survey the entire field. It is not intended as a practitioner's manual, so readers will not find cookbook recipes for addressing problems. Instead, the book emphasises basic principles and concepts, logical justifications, and analytical and problem-solving skills development. It features many examples that gradually increase in complexity.