High-Energy Gamma-ray Sources and the Quest for their Identification

The 3rd EGRET catalogue lists 271 sources and provides an identification for 101 of them. Thus, the unidentified sources are the majority in the gamma-ray sky, as we currently know it. Solving the mystery of the unidentified gamma-ray sources is a major challenge for all fields of astronomy. Here we shall review the different approaches towards source identification as well as the results achieved so far.