GEMs (Gas Electron Multipliers) are versatile ionising particle detectors that find applications in several fields, such as high energy physics and medical physics. In this book, the author analyses the properties of these devices, starting from temporal gain stability in sight of a possible application of GEM detectors as read-out of Time Projection Chambers. In addition, this study illustrates the development of a GEM-based detector prototype for in-beam Positron Emission Tomography. The author thanks Dr. Leszek Ropelewski, Prof. Fabio Sauli and Prof. Stefano Ragazzi for their helpful collaboration on this work.