This paper will describe a digital simulation model to assess the effectiveness of infrared imaging seeking missiles in presence of background and countermeasures. The model reproduces the infrared (IR) scene observed by the imaging seeker and generates the images at the detector output. The model replicates the image processing for automatic detection of potential targets in the seeker's field of view (FOV), and the target selection by the tracking algorithm. A model of a two-degree-of-freedom gyro stabilized seeker platform is also presented. The seeker platform is driven by the tracking algorithm to reduce the error between the seeker axis and the line of sight. The simulation also includes models for the missile aerodynamics in six degrees of freedom, the missile guidance and control system and the target and missile trajectories. As an illustration of the model, some results of simulation are shown for a target engaged by an infrared seeking missile using an imaging sensor.