What makes a gamma-ray source "unidentified?" Typical gamma-ray source error boxes are large by astronomical standards; therefore a positional correlation with a known object is usually not enough to identify a source. Historically, identifications have come in several ways: (1) finding a time- variable signal seen at another wavelength; (2) repeatedly seeing within gamma-ray error boxes members of a small, well-defined class of energetic sources; (3) seeing spatially-resolved gamma-ray emission from an extended source; and (4) finding a positionally-correlated, highly-unusual object with special parameters that might be expected to produce gamma rays.