Detectors are extremely useful in detection of radiations and capturing the processes taking place in experimental laboratory or in nature. These are available in a wide range depending on applications and environmental parameters. These are not only restricted for the application in the particle physic but also used for the medical applications, power industry, geological investigations, astrophysics...etc. Flexibility in design and portability of the detectors has made it very famous and handy to use for different purposes. Excellent example is "Geiger counter wristwatch, used for radiation detection. The development of detectors is a very challenging task, it takes a lot of R&D to characterize and ensure the performance in realistic situations. This book deals with the Gas Electron Multiplier(GEM) detector concept and development at CERN. A brief information about the atoms, radiations and their detection, fundamental particles, LHC accelerator and its detectors, makes it complete text to start with. The work is basically meant for students and beginners requiring basic knowledge. I wish them a very good luck and urge for their comments in order to improve the work.