The gamma-ray properties of unidentified EGRET sources

Although the majority of gamma-ray sources still remain unidentified, we have various kinds of information to characterize the observational properties of unidentified EGRET sources. Despite astronomical properties like locations of individual sources or the collective arrangement of the class as such, the nine years of CGRO observations provide the ability to investigate flux variability at different timescales, enable us to perform periodicity searches, determine gamma-ray source spectra between 30 MeV and 10 GeV and even investigate spectral variability. The basic observational properties of unidentified high-energy gamma-ray sources discovered by EGRET are reviewed. Various instrumental and observational peculiarities affecting the interpretation of the EGRET data are pointed out, also describing the way such biases might affect scientific conclusions drawn from the EGRET data.