LISP is an upgrade to the architecture of the IP internetworking system, one which separates location and identity (previously intermingled in IP addresses). This is a change which has been identified by the IRTF as a critically necessary evolutionary architectural step for the Internet. In LISP, nodes have both a 'locator' (a name which says _where_ in the network's connectivity structure the node is) and an 'identifier' (a name which provides a persistent handle for the node). A node may have more than one locator, or its locator may change over time (e.g. if the node is mobile), but it keeps the same identifier. One of the chief novelties of LISP, compared to other proposals for the separation of location and identity, is its approach to deploying this upgrade. LISP aims to achieve the near-ubiquitous deployment necessary for maximum exploitation of an architectural upgrade by i) minimizing the amount of change needed (existing hosts and routers can operate unmodified); and ii) by providing significant benefits to early adopters. This document is an introductory overview of the entire LISP system, for those who are unfamiliar with it. The first half of the document is a unified stand-alone brief introduction to LISP, for those who only want a basic understanding of LISP; the document taken as a whole provides a more detailed overview of LISP and its operation.