Abstract: Across the planet, the fear of physical attacks is experienced daily by millions of men and women, especially those walking or running alone in secluded areas. In many instances, attacks start from behind, exploiting a weakness in our forward-facing vision system. To address this, the present inventor devised, among other things, a rear-facing clip-on sensor that attaches to the back of a user, detecting rear-approaching objects or persons, and vibrating or beeping to alert the user. In some embodiments, the clip-on sensor includes a digital camera and a wireless transceiver to communicate with a smartphone or smartwatch, not only allowing the smartphone or smartwatch to alert the user via audible, visible, and/or tactile (haptic) alarms, but also to send SOS signals and real-time audio and/or video signals to designated persons or security services.