Fourth nerve palsy is a common cause of binocular vertical diplopia. However, it can be difficult to diagnose because the clinical signs are often subtle. In this chapter, we begin by reviewing the symptoms of fourth nerve palsy. We next discuss clinical maneuvers for diagnosing fourth nerve palsy, including the Parks-Bielschowsky three-step test. We list the common causes for isolated fourth nerve palsy and review its differential diagnosis, which includes skew deviation and myasthenia gravis. We then discuss the indications for neuroimaging in a patient with fourth nerve palsy. Lastly, we discuss management options for fourth nerve palsy, which include prisms and strabismus surgery.