It is now commonly accepted that N. Goodman's predicate “grue” presents the theory of confirmation of C. G. Hempel (and other such theories) with grave difficulties. The precise nature and status of these “difficulties” has, however, never been made clear. In this paper it is argued that it is very unlikely that “grue” raises any formal difficulties for Hempel and appearances to the contrary are examined, rejected and an explanation of their intuitive appeal offered. However “grue” is shown to raise an informal, “over-arching” difficulty of great magnitude for all theories of confirmation, including Hempel's theory.