Osteonecrosis of the knee was first described by Ahlback in 1968. Today, it is classified as spontaneous and secondary osteonecrosis. Spontaneous osteonecrosis is typically seen with unilateral involvement in patients older than 60 years. Secondary osteonecrosis occurs in younger patients bilaterally and with multifocal involvement. Although its etiology is still unclear, associated factors such as steroid therapy, alcoholism, and some chronic inflammatory diseases are well-known. Many treatment modalities have been reported, including conservative treatment, arthroscopic debridement, core decompression, microfracture, autologous osteochondral transplantation, chondrocyte culture, high tibial osteotomy, and arthroplasty. Conservative treatment and observation are generally accepted at any stage. For surgical treatment, all procedures are applied to both types of osteonecrosis.