Objectives: To review the experimental evidence and development of concepts in knee kinematics and to present a synthesis of current theories. Data Sources: Historical literature from private collections and published journals, from Galen in 160 AD, and Weber and Weber in 1860, through to current research in knee kinematics, sourced through MEDLINE and CINAHL. Study Selection: Studies of the healthy human knee in vivo and in vitro were included. Other studies were included when relevant, for example, when knee surgery methods have led to a change in kinematic concepts. Of 285 items, 94 were included based on their contribution to original research. When relevant, authors were contacted to resolve issues. Data Extraction: Sources included were descriptive studies, anatomic dissections, controlled experimental designs, editorials, and review articles. Data Synthesis: The axes of rotation of the knee are fundamental to kinematic models. The hinge model is contradicted by the ellipsoid shape of the femoral condyles, which results in a moving instant center of motion. However, the "instant center of motion" model was based on analysis of sagittal sections, oblique to the plane of movement and neglecting rotation. The four-bar linkage theory linked cruciate ligament isometry with the roll and glide pattern of knee motion. Recently, however, studies of the biomechanics and histology of the knee ligaments have enabled more accurate kinematic modeling. Three-dimensional imaging and computer modeling have made possible analysis of kinematics parallel to the planes of motion and incorporation of conjoint rotation. Femoral roll back is now described as the manifestation of longitudinal rotation during knee flexion. Conclusions: Current research concludes that the knee has 4 independent axes: patella, posterior condylar, distal condylar, and longitudinal axes. The axes combine to produce the characteristic helical motion of the knee.