Arterial stiffness is highly relevant to cardiovascular disease. Arterial stiffness is central to the pathogenesis of isolated systolic hypertension and directly impacts left ventricular afterload, pressure pulsatility in the arterial tree, and its penetration into the microvasculature of target organs such as the brain and kidney. Arterial stiffness is affected by various risk factors and biologic processes. Measurements of arterial stiffness may therefore not only provide information about prevalent processes, but also valuable information regarding the cumulative history of risk factor exposure. Available studies consistently demonstrate that large artery stiffness, measured via carotid-femoral pulse wave velocity, independently predicts the risk of incident cardiovascular events in clinical and community-based cohorts. Understanding the basic principles and definitions related to arterial stiffness is therefore desirable for cardiovascular clinicians and researchers. This introductory paper reviews basic physical principles and definitions regarding arterial stiffness and the most important non-invasive methods for its quantification in vivo.