This lecture covers the principles and technologies behind large-area sensors for X-ray detection, including direct and indirect detection methods using semiconductor diodes and scintillators. It discusses the performance metrics of various X-ray detectors, such as conversion efficiency and refractive index. The lecture also explores the use of a-Se and CdTe as direct conversion materials, as well as the application of perovskites for enhanced X-ray sensitivity. Additionally, it delves into the design and integration of active matrix flat panel imagers for radiography, highlighting the advantages of a-Si:H and a-Se based detectors for different imaging modalities.