The knee is the largest joint in the human body, making it particularly vulnerable to injury. Acute injury can result in ligamentous sprains and strains, dislocations/subluxations, and/or bony fractures either from direct trauma to the joint or indirect stress on previously healthy tissue. Most injuries to the knee involve the surrounding ligaments and/or tendons, with fractures composing an exceedingly small percentage of injuries. Prepubescent children are at greater risk for bony injury because before closure of the physis, tensile bone strength is less than that of ligament and tendon attachments. Once the physis closes, bone tensile strength exceeds that of ligaments/tendons, making ligamentous injury more common in postpubertal children. Although not the subject herein, it is important to remember that aside from traumatic injury, knee pain can be a manifestation of infectious, rheumatic, or oncologic pathology.Sprains or tears of the anterior or posterior cruciate ligament or the medial or lateral collateral ligament, meniscal tears, quadriceps tendon ruptures, patellar tendon ruptures, and hamstring strains are soft tissue injuries that can result from acute trauma to the knee.Anterior cruciate ligament tears are the most common ligamentous knee injury in children and skeletally immature adolescents. Typically, they occur during a sudden change in the rotational forces on the knee when the foot is fixed. Patients will describe a “pop,” or their knee “giving out,” and present with an acutely swollen knee with an effusion. Posterior cruciate ligament injuries are rare, resulting from direct force on the tibial tubercle, which pushes the tibia posteriorly relative to the femur. Medial and lateral collateral ligament injuries are more common in the skeletally mature patient and result from direct trauma to the contralateral side of the knee: trauma at the lateral aspect causes medial collateral ligament injury, and medial trauma affects the lateral collateral ligament. (A list of maneuvers to assess ligamentous injury appears in Table 1.)Meniscal tears result from sudden twisting of the leg while bearing weight. Patients, most commonly postpubertal, often describe their injury as a painful “locked knee” or “torn cartilage.” Quadriceps and patellar tendon ruptures occur in postpubertal children, typically older athletes, with an abrupt contraction of the quadriceps muscles, as when landing from a high jump or with a sudden change in direction at high velocity. As a result of the rupture, extension of the knee is limited.The muscles most affected by an acute injury to the knee are the hamstrings. Diagnosis of a hamstring strain is clinical, manifesting with significant swelling, local tenderness, and sometimes overlying ecchymosis.Dislocations of the knee joint in children are infrequent but serious limb-threatening injuries, occurring most frequently after physis closure. Typically, a significant traumatic force is involved, resulting in an overt deformity. Because the dislocation can shear the popliteal artery, which is anchored firmly to either end of the joint, and can also disrupt the peroneal nerve, evaluation of the limb’s neurovascular status is a crucial part of the physical examination.More common than dislocations of the knee joint, patellar dislocations occur with forceful contraction of the quadriceps while the lower leg is abducted. Classically, a “popping” sensation is described, and the patient has significant pain with the hip abducted, the knee held in the flexed position, and the patella laterally displaced. Because the diagnosis is based on history and examination, the dislocation can be reduced before obtaining radiographs by placing gentle pressure on the lateral aspect of the patella as the patient extends the knee. Postreduction radiographs should be obtained to rule out an associated avulsion or patella osteochondral fracture. If the history suggests patellar dislocation but the patella is in anatomical position, the dislocation could have self-reduced or subluxated. Such patients can have slight medial patella pain and are anxious during an attempt to push the patella laterally (apprehension test). If subluxation is suspected, radiographs should be obtained to look for an associated osteochondral fracture.Young children and other skeletally immature patients are the most likely to sustain Salter-Harris–type physeal fractures from direct trauma, as in contact sports or with motor vehicle accidents. Physeal fractures most commonly occur at the distal femoral epiphysis. Although rare, proximal tibial epiphyseal fractures can be limb-threatening because the popliteal artery lies just posterior to the tibial epiphysis. Sudden directional change can lead to tibial spine (intercondylar eminence) avulsion fractures in children 6 to 16 years of age because the tibial spine is not fully ossified and thus weaker than the surrounding ligaments. Tibial tuberosity avulsions occur most often in adolescents when the knee’s extensor mechanism is challenged during sudden acceleration or deceleration, typically with jumping. The tibial tubercle becomes swollen and painful, and the patient is unable to fully extend the knee or perform a straight leg raise. Patella fractures occur after direct trauma to the patella, such as falling on a flexed knee. They are rare in young children because the patella does not ossify until 3 to 6 years of age. Patellar avulsion fractures are more common, resulting from forceful contraction of the quadriceps. The patella is swollen, tender, and painful with extension. Osteochondral fractures can accompany patellar dislocation or injury to a ligament or meniscus. If left untreated, they can lead to permanent defects and osteoarthritis. Nearly all fractures of the knee are associated with swelling, significant pain, limited range of joint movement, and sometimes hemarthrosis.Subacute overuse injuries can present with chronic pain or acute worsening of pain. Table 2 lists the common overuse injuries.Evaluation of an injury to the knee includes an attempt to locate point tenderness, assessment of range of motion and ligamentous laxity, presence of an effusion, and neurovascular status of the lower leg. Laxity of the knee should be assessed in full extension and at 30° of flexion. The Ottawa Knee Rule is a highly sensitive clinical decision tool to help guide the need for knee radiographs in the setting of injury without a gross deformity. Anteroposterior and lateral radiographs of the knee (and a patellar view if indicated) should be obtained only if any of the following is present: patellar tenderness, tenderness at the head of the fibula, inability to flex to 90°, and inability to bear weight for 4 steps (regardless of limping), immediately with the injury and at presentation.Avulsion fractures of the patella pole (patellar sleeve fracture) can be missed easily on radiographs, and diagnosis may necessitate more advanced imaging. Hip imaging should also be considered after careful examination because hip pathology frequently causes referred knee pain. If knee radiographs do not reveal a fracture but the knee remains painful and swollen, damage to a ligament or meniscus is likely. Emergent orthopedic consultation is indicated if neurovascular compromise is suspected, puncture or lacerations raise concern for violation of joint integrity, with laxity of the knee in any direction, and with nonlateral patellar dislocations.Definitive management depends on the injury and ranges from conservative treatment with rest, ice, compression, and elevation to urgent operative intervention. Weightbearing status varies with the type of injury, and analgesia and ambulatory aides should be provided as necessary. If not indicated at the time of presentation, orthopedic follow-up should occur within 1 to 2 weeks if symptoms do not improve.Our authors note that aside from trauma, knee pain can result from infection or from rheumatoid and oncologic disorders. Another category in the differential diagnosis is joint laxity or hypermobility, which is, in fact, a relatively common cause of recurrent knee pain in children. Genetic disorders of connective tissue, such as Marfan and Ehlers-Danlos syndromes, as well as syndromes such as trisomy 21 and William syndrome, are associated with joint laxity, but much more common than any of these is the so-called benign joint hypermobility syndrome (BJHS). More frequent in girls than in boys, possibly related to estrogen effects, and especially among children of African or Asian descent, BJHS probably reflects excessive ligamentous laxity that allows range of motion beyond the normal. The knee joint is particularly susceptible, especially in the first decade after birth. Affected children may be described as “double jointed,” and their pain typically occurs late in a day marked by vigorous exercise. The painful joint(s) can show swelling, but not the redness and heat typical of inflammation. Recognition of BJHS, aided by the Beighton 9-point scale for hypermobility, can avoid unnecessary diagnostic evaluation and expedite appropriate management: reassurance with patient education, muscle-strengthening exercises, and pain control.