The Kellgren-Lawrence (KL) classification system is a widely used radiographic tool for diagnosing osteoarthritis (OA), particularly in the knee. Developed by Kellgren and Lawrence in 1957, the KL classification assigns grades from 0 to 4 based on radiographic findings, with Grade 0 indicating no OA and Grade 4 indicating severe OA. The system is applied to knee radiographs and is used in clinical practice and research to assess the severity of OA. It has been validated for inter- and intraobserver reliability, with the knee showing a high correlation coefficient of 0.83. However, the KL classification has limitations, including its assumption of linear disease progression and insensitivity to change. It may not accurately reflect the physical state of the joint and has been criticized for not accounting for patellofemoral arthritis. Additionally, there are inconsistencies in the original description of the classification and its application in subsequent studies, leading to confusion among researchers. Despite these limitations, the KL classification remains widely used, often in conjunction with clinical assessments. Recent studies suggest that the KL classification has high diagnostic accuracy, but further research is needed to develop treatment algorithms based on classification grades to improve clinical decision-making.The Kellgren-Lawrence (KL) classification system is a widely used radiographic tool for diagnosing osteoarthritis (OA), particularly in the knee. Developed by Kellgren and Lawrence in 1957, the KL classification assigns grades from 0 to 4 based on radiographic findings, with Grade 0 indicating no OA and Grade 4 indicating severe OA. The system is applied to knee radiographs and is used in clinical practice and research to assess the severity of OA. It has been validated for inter- and intraobserver reliability, with the knee showing a high correlation coefficient of 0.83. However, the KL classification has limitations, including its assumption of linear disease progression and insensitivity to change. It may not accurately reflect the physical state of the joint and has been criticized for not accounting for patellofemoral arthritis. Additionally, there are inconsistencies in the original description of the classification and its application in subsequent studies, leading to confusion among researchers. Despite these limitations, the KL classification remains widely used, often in conjunction with clinical assessments. Recent studies suggest that the KL classification has high diagnostic accuracy, but further research is needed to develop treatment algorithms based on classification grades to improve clinical decision-making.