On the origin of unidentified EGRET gamma-ray sources

The identification of celestial gamma-ray Sources with astronomical objects or object classes has remained the initial and most fundamental key for understanding their physical nature. The observational characteristic of a gamma-ray emitter and the conditions under which an astronomical object is able to produce energetic garnma-ray emission defines the range of candidates available for source identifications. The main obstacle must be seen in the fact that a gamma-ray source location is often imprecise, a flux history could only be established on the basis of weeks, and uncertainties in the gamma-ray observables are considerably large. Therefore coordinated multifrequency follow-up campaigns or spatial-statistical methods are required to assign proper counterpart identifications. Although Active Galactic Nuclei and pulsars are uniquely identified EGRET sources, many other garnma-ray sources still remain unidentified. I will review properties of the population and highlight the characteristics of potential counterparts of the Stil I unidentified gamma-ray sources detected by EGRET.