Hybrid pixel sensors (detectors) have shown to be a valid alternative to other types of Xray imaging devices due to their high sensitivity, linear behavior and wide dynamic range, and low noise. One important feature of these devices is the fact that detectors and readout electronics are manufactured separately. The charge created by the interaction of X-ray photons in the sensor is very small and has to be amplified in a low-noise circuit before any further signal processing. The signal induced on the electrodes of the sensor is transferred to the readout chip, where it is integrated in a charge sensitive amplifier. The issue reviews on physical principles of operation and design of the hybrid pixel sensors developed on the basis of the silicon CMOS and GaAs MESFETtechnologies. The authors have designed GaAs charge sensitive amplifiers for hybrid pixel detectors and show the results of a simulation.