The development of magnetic sensors for the detection, localization, and classification of time-critical targets is of great importance in monitoring, surveillance, intelligence, and security applications. This is particularly true for the military where precision targeting of armed enemy troops, tracked and wheeled vehicles requires timely updates of their movements. To address this need, Quantum Magnetics (QM) is developing small, low power, low cost magnetic sensor modules, and high performance digital electronics that can be used to passively detect magnetic anomalies in the battlefield generated by the presence and movement of armed troops and military vehicles. The focus is on Magneto-Resistive (MR) sensors that can be fabricated by microelectronics techniques. These sensors represent a mature technology and are widely available commercially. They operate at room temperature with high sensitivity and have a broad bandwidth. The long-term vision is to integrate these modules into a network of battlefield microsensors that include a variety of other sensing technologies (acoustic, seismic, IR, etc.). We will discuss MR system design considerations and results obtained in recent field tests. A highly sensitive magnetic sensor module would also find numerous applications in security operations and surveillance of perimeters and borders, landmine/UXO detection, and detection of concealed weapons.