Ultrasound of the knee is best suited for the evaluation of tendons, fluid collections and effusion, synovitis, periarticular soft tissue masses, muscles, and the collateral ligaments. Advantages of sonographic evaluation include the lack of ionizing radiation, the noninvasive nature of the examination, focused evaluation correlated with the specific site of pain, multiplanar capability, and utility in patients for whom magnetic resonance imaging (MRI) is contraindicated. The ability to visualize the microanatomy of tendons, ligaments, nerves, and muscles is also an advantage over MRI. Moreover, ultrasound is generally readily accessible and of lower cost than MRI. Ultrasound is less suited for the evaluation of internal derangement. Certain structures that are exquisitely evaluated by MRI, including the menisci, cruciate ligaments, bone marrow, and articular cartilage, are suboptimally evaluated by sonography. This article reviews the structures and pathological processes for which ultrasound has been shown useful, emphasizing appropriate technique and normal anatomy, appearance of pathological processes, and advantages and limitations of sonographic evaluation.