Emerging 5G and future 6G devices are expected to leverage newly-available mmWave spectrum above 20 GHz to enable cutting-edge applications such as high-quality wireless virtual and augmented reality (VR/AR), connected autonomous transportation, and cloud-controlled robotics. I enable reliable, low-latency, and high-throughput communications for indoor applications such as VR/AR and cloud robotics by using commodity lidar devices to map environmental reflectors. I demonstrate that this approach reduces beam-selection overhead by over 95% in indoor multi-user scenarios, reducing network latency by over 80% and increasing throughput by over 2x in mobile scenarios. To enable reliable autonomous sensing, I develop a series of techniques that enable sensors to look around environmental blockages using multipath reflections. Environmental blockages cause a large number of traffic collisions, being the primary factor in around 20% of crashes at intersections. By leveraging irregularly-shaped ambient reflectors already in the environment, we show a > 3× improvement in radar coverage of hidden regions in real urban scenarios, compared to state-of-the-art around-corner radar systems. We further develop cheap, rugged, fully-passive metasurface systems that are placed near sightline obstructions to enable high-resolution sensing of hidden objects. In real-world tests, this system has a 3× larger field of view compared to an equivalent-size planar reflector, without significantly impacting the localization accuracy.