With the advances in deep submicron CMOS technologies, CMOS-based active-pixel sensors (APS) have become a practical alternative to charge-coupled devices (CCD) imaging technology. Key advantages of CMOS image sensors are that they are fabricated in standard CMOS technologies, which allow full integration of the image sensor along with the analog and digital processing and control circuits on the same chip and that they are of low cost. Since there is a practical limit on the minimum pixel size (4~5 mum), then CMOS technology scaling can allow for an increased number of transistors to be integrated into the pixel. Such smart pixels truly show the potential of CMOS technology in imaging applications, especially for high-speed applications. This work discusses various active-pixel sensors (APS) and shows the feasibility of using the DC-level to increase the sensitivity of the pixel for low-level light applications. Avalanche-photodiodes (APDs) are described, in addition to a discussion of the breakdown mechanism and microplasma in avalanche breakdown for single photon APDs. A fully integrated, 16 times 16 pixel CMOS camera-on-a-chip, fabricated in a standard CMOS 0.18 mum technology is also shown in this work. The array is based on 256 APS with a pixel size of 20 mum times 30 mum, a fill-factor of 60% with all digital and analog blocks implemented on-chip.