Osteoarthritis (OA) is one of the most common forms of degenerative joint disease and causes of disability. Although the recognition of OA has steadily increased over recent decades, current treatments mainly focuse on symptomatic relief, and no efficacious medical intervention is yet available to control the progression of this disease. With disease progression, the whole joint structure will be damaged gradually and irreversibly in the end point of OA. There is an increasing awareness on the importance in identifying early phases of the degenerative processes in knee OA, the crucial period of the disease when there might still be the possibility to initiate treatments preventing its progression. The purpose of this paper was to provide an updated review of the current status of the diagnosis and classification of early knee OA, including the clinical, radiographical, histological, MRI, and arthroscopic definitions and biomarkers. A further exploration on practical classification criteria of early knee OA will provide new insights and technologies for the early prevention and detection of osteoarthritis, in the hope of providing help for the early treatment of OA.