This chapter provides a detailed discussion of gas-filled detectors (Geiger-Muller counters and gas-discharge proportional counters), scintillation and semiconductor detectors. The main characteristics (energy resolution, detection efficiency, and time resolution) of X-ray detectors are demonstrated. Gas proportional and scintillation detectors are widely used in modern wavelength-dispersive X-ray fluorescence (WDXRF) spectrometers. Semiconductor detectors have found wide application in modern energy-dispersive X-ray fluorescence (EDXRF) and total-reflection X-ray fluorescence (TXRF) spectrometers, as well as in EDXRF spectrometers, which are part of X-ray microprobe analysis systems and scanning electron microscopes. These microscopes are equipped with secondary electron (SE) detectors, backscattered electron (BSE) detectors, cathodoluminescence (CL) detectors in addition to semiconductor X-ray detectors. This is demonstrated using the example of a scanning electron microscope MIRA3 LMH produced by TESCAN, Czech Republic. Beam Deceleration Technology was implemented in these microscopes, which is designed to improve image quality at low accelerating voltages. Today, three large groups of semiconductor detectors used in X-ray imaging systems can be distinguished, which are the result of the development of different technological approaches to image formation (Coupled Charge Device-, Thin Film Transistors-, Complementary Metal Oxide Semiconductor-Technologies).