Although the majority of gamma-ray sources still remain unidentified, we have various kinds of information to characterize the basic properties of unidentified EGRET sources. Despite astronomical properties like locations of individual sources or the arrangement of the class as collective, the nine years of CGRO observations provide the ability to investigate time variability at different timescales. The EGRET data enable us to determine gamma-ray spectra between 30 MeV and 10 GeV. Both spectrum and flux variability are reviewed and discussed in order to provide a fundament for the interpretation of unidentified sources on the basis of observational data.