  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.
