Historically, the trochlear (IV) nerve has been "neglected" by neurologists and ophthalmologists. However, the reported incidence of trochlear palsy in two large series has more than doubled in the past two decades, indicating increasing awareness of this nerve. Trauma is the most common cause of trochlear palsy, as the trochlear nerve is anatomically more vulnerable to trauma than the other ocular motor nerves. Trochlear palsy can also be caused by vascular and inflammatory diseases, congenital factors, toxic substances and tumors. Diplopia secondary to vertical and horizontal deviation is the most common presentation. The trochlear nerve has a relatively high recovery rate after the underlying cause of injury has been corrected. In this article, the anatomy and physiology of the trochlear nerve are described, and the various etiologies, methods of diagnosis and differential diagnosis of trochlear palsy are reviewed.