This chapter provides an outline of the main concepts and issues related to optical packet switching (OPS). The basic network functions required by this paradigm are discussed and references to past and current research on the topic are provided. Optical packet switching is a transport technique that assumes information to be organized in packets formed by a payload and a header both encoded as optical signals. The payload is transferred through the network without any optical to electronic conversion. The header is processed in the early phase in the electronic domain. Optical packet switching may be considered as a long-term and more flexible alternative to the circuit-switched optical networks currently being deployed by operators. This innovative paradigm aims at optimizing the utilization of the dense wavelength division multiplexing (DWDM) channels by means of fast and highly dynamic resource allocation, overcoming the inefficiency typical of the circuit transfer modes. Traditionally, packet transfer modes have proved to be very flexible by nature, with respect to bandwidth exploitation. In fact, link capacity is shared in time by means of statistical multiplexing, while contentions occurring at each node are solved by storing packets as long as the required resources become available again. Therefore, network links are in general used more efficiently in OPS than in circuit-switching.