  More than half the sources in the Third EGRET (3EG) catalog have no firmly
established counterparts at other wavelengths and are unidentified. Some of
these unidentified sources have remained a mystery since the first surveys of
the gamma-ray sky with the COS-B satellite. The unidentified sources generally
have large error circles, and finding counterparts has often been a challenging
job. A multiwavelength approach, using X-ray, optical, and radio data, is often
needed to understand the nature of these sources. This chapter reviews the
technique of identification of EGRET sources using multiwavelength studies of
the gamma-ray fields.
