The Geigermode Avalanche Photodiode (G-APD) is a novel solid state photosensor with high internal gain. G-APDs have a high photon detection efficiency, can detect single photons, have a high internal gain and are ultracompact. It has the potential to replace Photomultipliers in many research applications and in nuclear medical detectors. In the first part of the talk the main parameters and the performance of G-APDs will be reported. In the second part some potential applications will be discussed. Special emphasis will be put on two applications: a) detection of Cherenkov light from cosmic air showers and b) first tests for a use in PET (Positron emissiontomography).