This paper explores the nature of phonological opacity (in the sense of Kiparsky 1971, 1973) within Optimality Theory. Previous attempts to address opacity in OT are discussed and a novel proposal, an inter-candidate faithfulness relation called 'sympathy', is offered. Specific applications of sympathy are presented and some general results are derived about counter-bleeding, counter-feeding, multi-process, and Duke-of-York opaque interactions.