The keys to functional endoscopic sinus surgery are an understanding of the underlying mechanisms of paranasal sinus disease and the ability to diagnose the causes accurately. Previously unrecognized causes of recurrent acute sinusitis and of chronic sinus symptomatology can now be identified by careful diagnostic evaluation. Systematic nasal endoscopy and high-resolution computed tomographic imaging provide complementary diagnostic information that can allow for the recognition of problems not identifiable by other means. The ability to diagnose these problems and to correct them with functional endoscopic surgery heralds new possibilities in the field.