This book gives an overview of Multicore architectures, how they derive from multiprocessors, and illustrates the new applications they enable. A multicore processor has multiple cpu and memory elements in a single chip. Being on a single chip reduces the communications times between elements, and allows for multiprocessing. Advances in microelectronics fabrication techniques lead to the implementation of multicores for desktop and server machines around 2007. It was becoming increasingly difficult to increase clock speeds, so the obvious approach was to turn to parallelism. Currently, in this market, quad-core, 6-core, and 8-core chips are available. Besides additional cpu's, additional on-chip memory must be added, usually in the form of memory caches, to keep the processors fed with instructions and data. There is no inherent difference in multicore architectures and multiprocessing with single core chips, except in the speed of communications. The standard interconnect technologies used in multiprocessing and clustering are applied to inter-core communications. Multicore technology is mainstream, and enables a vast application space.