: This article begins with a discussion of persistent vegetative state (PVS), focusing on concerns related to both diagnosis and prognosis and paying special attention to the 1994 Multi-Society Task Force report on the medical aspects of PVS. The article explores the impact of diagnostic and prognostic uncertainties on prospective thinking regarding the possibility of PVS and considers the closely related question of how prospective thinkers might craft advance directives in order to deal most effectively with this possibility.