Tacrolimus (FK506) has been used successfully as a systemic immunomodulator for more than 2 decades, and numerous studies have investigated its mechanisms of action. Systemic and topical tacrolimus have been investigated as treatments for ocular surface disorders that may have an immune-based inflammatory component. In these studies, tacrolimus has shown efficacy in corneal graft rejection, inflammatory conjunctival and corneal diseases, uveitis, and graft-versus-host disease. As these disorders are often refractory to other available treatments, ophthalmic or systemic tacrolimus is a welcome nontoxic adjunct or replacement to potentially toxic topical or systemic immunosuppressive therapies.