Ultrasonography is an imaging modality that facilitates the diagnosis of pathology and injection therapy without exposing the patient to radiation. In addition, ultrasonography has become popular because of its portability, low cost, and production of real-time tomographic images that provide a cross-sectional view of anatomic structures. Despite its benefits and widespread adoption in general medicine and other specialties, however, ultrasonography is not as well adapted as a diagnostic and research tool in orthopaedic surgery. An understanding of the basic principles of ultrasonography and the evidence supporting its use can aid the orthopaedic surgeon in applying this modality appropriately in clinical practice.