An autonomous image sensor for real time target detection and tracking is presented. The sensor is based on a CMOS APS array, equipped with in-pixel functionality and integrates analog and digital components to achieve autonomous operation with minimal power dissipation. The system employs a two-phased operation flow; during the initial acquisition stage, the digital controller detects and acquires the brightest targets in the field of view within a single frame and defines windows of interest (WOI) around the center of mass coordinates of each object. Subsequently, the system moves into the analog tracking mode during which all areas outside of the WOI are entirely shut down, thus saving power to a number of orders of magnitude. In addition to its low power dissipation, the sensor features real-time operation, low fixed pattern noise, linearity and the ability to track a predefined number of targets throughout the entire field of view. A 64x64 pixel sensor array has been designed in 0.18µm CMOS technology and is operated via a 1.8V supply. The imager architecture is discussed, the circuits' descriptions are shown and simulation results are presented.