Today, Complementary-Metal-Oxide-Semiconductor (CMOS) Image Sensors (CIS), also called Active Pixel Sensors (APS), are the most popular imager technology with several billions manufactured every year. They represent about 90% of the imager market and should exceed 95% in a couple of years. Compared to the main alternative imager technology, the Charge Coupled Device (CCD), CISs have several major benefits such as low-power consumption, high-integration, high speed and the capacity to integrate advanced CMOS functions on-chip (and even inside the pixel). Thanks to the latest technology innovations, CISs are now matching the performances of CCDs in terms of image quality and sensitivity placing them at the forefront even in high-end applications such as digital single-lens reflex, scientific instruments, and machine vision. Thanks to these advantages, CISs are also used in harsh radiation environment for applications such as: space applications, X-ray medical imaging, electron microscopy, nuclear facility monitoring and remote handling (nuclear power plants, nuclear waste repositories, nuclear physics facilities…), particle detection and imaging, military applications etc.. Designing, hardening and testing a sensor for such applications require the understanding of the CIS behavior when exposed to radiation sources. Understanding and improving further the intrinsically good radiation hardness of APS has been a topic of interest since its invention. This interest has been recently growing with the coming of new behaviors brought by the profound evolution of CIS technologies (as discussed throughout this manuscript) compared to the older generation mainstream CMOS processes used in early work. The aim of this chapter is to give an overview of the parasitic effects that can undergo a modern CIS when it is exposed to a high energy particle radiation field.