Abstract: This paper introduces the electro-optic (EO) multifunction sensor concept for military applications and describes why it would be advantageous to merge several EO functionalities, possibly not as performance-oriented as single task sensors, into a unique system to create performance synergies by acting together. The paper conveys the concept of several known capabilities merged into a single-aperture sensor to enhance threat detection (TD), situational awareness (SA), and targeting capabilities. Sensors with tremendous sensitivity and dynamic range are currently available, and when coupled with properly engineered optics and processing techniques, additional synergies can be developed in order to provide a key technological advantage when exploited in a military context. In this paper, the drivers, basic concepts, and challenges are introduced to grasp the essence of EO multifunction sensors. The research fields that may present breakthroughs are presented in conjunction with examples. Finally, current work from the industry and government agencies is briefly discussed.