Subchondral Insufficiency Fracture of the Knee (SIFK), historically termed Spontaneous Osteonecrosis of the Knee (SPONK/SONK), represents a clinically significant and often underdiagnosed cause of knee pain in older adults. The subchondral bone plays a critical role in load distribution, maintenance of articular cartilage integrity and preservation of joint congruity, and its structural failure can precipitate acute pain, subchondral collapse, and secondary osteoarthritis. Traditionally considered an idiopathic ischemic necrosis, advances in magnetic resonance imaging, histopathology and biomechanics have redefined SIFK as a mechanical insufficiency fracture in osteopenic or biomechanically compromised bone, often exacerbated by meniscal pathology, particularly posterior root tears and extrusion, and varus malalignment. Lesion progression may range from spontaneous resolution in small, early stage fractures to subchondral collapse and secondary osteonecrosis in advanced cases, with histopathology demonstrating that necrosis is typically a secondary phenomenon, whereas subchondral fracture represents the primary pathology. SIFK predominantly affects older adults, with a marked female predominance, most commonly involving the medial femoral condyle. Clinically, presents with sudden-onset, well-localized medial knee pain, frequently mimicking meniscal injury, with nocturnal exacerbation and weight-bearing intolerance. Physical examination typically demonstrates focal tenderness over the medial femoral condyle, with mild synovitis or effusion, while overall knee range of motion and stability are generally preserved. Radiographs are frequently normal in early stages, whereas advanced lesions exhibit subchondral lucency or condylar flattening. Magnetic resonance imaging (MRI) represents the diagnostic gold standard, characteristically demonstrating a hypointense subchondral fracture line, extensive bone marrow edema, and often concomitant meniscal pathology. Differential diagnosis includes secondary osteonecrosis, osteoarthritis, traumatic fractures and transient bone marrow edema syndrome. Prognostic factors include lesion size, subchondral deformity, presence of a fluid-filled cleft, meniscal extrusion and baseline osteoarthritis grade. Radiographic (Koshino, Aglietti) and MRI-based grading systems provide essential guidance for lesion staging, prognostic assessment, and therapeutic decision-making. Small, early stage lesions generally respond to conservative management, including protected weight-bearing, analgesics, nonsteroidal anti-inflammatory drugs, and pharmacologic therapy such as bisphosphonates. Joint-preserving surgical interventions, including arthroscopic debridement, core decompression, osteochondral grafting, and high tibial osteotomy, are indicated for symptomatic or progressive lesions prior to subchondral collapse. For advanced or extensive lesions, unicompartmental or total knee arthroplasty represents the definitive treatment, demonstrating high rates of clinical success and long-term implant survivorship. This chapter provides a comprehensive review of the epidemiology, pathophysiology, clinical presentation, imaging, classification, and evidence-based management of SIFK, integrating contemporary diagnostic and therapeutic paradigms to optimize patient outcomes.