Injury to the proximal interphalangeal (PIP) joint is one of the most common hand injuries encountered by orthopedic surgeons. The injuries are often misdiagnosed, resulting in delayed treatment and permanent impairment. The different types of PIP-joint fractures and dislocations have been well described, and a variety of therapeutic options have been proposed. Early detection and aggressive rehabilitation are the cornerstones of successful treatment. This is the first of a two-part series reviewing PIP injuries. We describe the anatomy and biomechanics of the PIP joint and review the classification and diagnosis of these injuries.