This paper describes and quantifies the benefits of adding low-overhead active messages to many-core, cache-coherent chip-multiprocessors. The active messages we analyze are user defined and trigger the atomic execution of a custom software handler at the destination. Programmers can use these active messages to both move data with less overhead than cache coherency and, more importantly, explicitly send computation to data. Doing so greatly improves (11× speed, 4.8× energy) communication idioms such as shared object modification, reductions, data walks, point-to-point communication, and all-to-all communication. Active messages enhance program scalability: applications using them run 63 percent faster with 11 percent less energy on 256 cores. The relative benefits of active messages grow with larger numbers of cores.