During this lecture, we will discuss the role of using prism to alleviate diplopia. We will describe the testing used to evaluate a diplopic patient. We will use a step-wise approach to investigate the etiology of the double vision which will determine what type of prism to utilize. The advantages of ground-in, Fresnel and occlusion will be addressed. Finally, cases will be used to illustrate how to prescribe prism for common neuro-ophthalmic and neurologic disorders.