The Laser Interferometer Space Antenna (LISA) is a space-based observatory that will detect gravitational waves from massive and distant sources that cannot be observed from the ground. This talk will introduce LISA’s main science goals, including mergers of massive black holes, compact binaries in our Galaxy, and what gravitational waves can reveal about the growth of structure over cosmic time. It will outline the mission concept and the basic principles of space-based laser interferometry, highlighting the challenge of measuring extremely small distance changes across millions of kilometres. The talk will conclude with an overview of LISA data analysis, describing how gravitational-wave signals are extracted from a rich and complex data set.