Read out circuit is a critical component needed for infrared and visible light imaging system. Advances in image sensors and microelectronics have led to the development of digital read out fabricated using deep sub micro integrated circuit techniques. An brief review is given of the image sensor developing. Major adaptation circuit are review including source follower, direct injection, and capacitance trans impedance amplifier. On chip analog signal processing and analog to digital conversion techniques are also discussed. Noise shaping modulation is a promising technique to build a low power, ultra small area, high precision pixel level AD convertor.