Diagnosis of posterolateral corner injuries can be challenging. Understanding the mechanism of injury in combination with careful history taking and a thorough physical examination are crucial. Clinical findings may be subtle, especially in the acute setting. Specialized tests such as external rotation recurvatum test, posterolateral drawer test, reverse pivot shift test, and dial test are particularly helpful. A characteristic radiographic finding is the arcuate sign, whereas medial Segond fractures can be associated with posterolateral corner injuries. Magnetic resonance imaging is most useful when performed with a high strength magnet utilizing a coronal oblique technique. Prompt and accurate diagnosis of posterolateral corner injuries allows the orthopedist to initiate the appropriate repair and reconstruction procedures to assure optimum treatment results.