Quantum networks (QNs) transmit quantum information between quantum devices and allow distribution of quantum entanglement, a physical resource known to be useful for quantum information processing. QNs in the form of internets or intranets enable larger quantum computations by connecting quantum computers together. Entangled sensor networks may enable precision metrology beyond what is possible with the best individual quantum sensors. Quantum properties can also be utilized to secure communication in novel ways. These promises of new science and technology motivate active research into creating and understanding QNs and their constituent components.