GENIE is a new neutrino event generator designed for experimental neutrino physics, aiming to develop a 'canonical' Monte Carlo generator that can simulate neutrino interactions with all nuclear targets and flavors from MeV to PeV energy scales. The focus is on the few-GeV energy range, which is crucial for long-baseline precision neutrino experiments using accelerator beams. GENIE is a large-scale software system, consisting of approximately 120,000 lines of C++ code, and has been widely adopted by neutrino experiments. It addresses unique challenges in neutrino simulations, such as the transition between non-perturbative and perturbative regimes, and supports the full life-cycle of simulation and analysis tasks. The article discusses the paradigm shift brought by GENIE in neutrino physics simulations, highlighting the challenges in simulating neutrino interactions over a broad energy range and the importance of accurate models for various physical processes. It also provides an overview of the physics models available in GENIE, including nuclear physics models, cross section models, and hadronization models. The article describes the object-oriented design of GENIE, its applications and utilities, and the structure of the GENIE collaboration. Additionally, it covers the tuning and validation of the physics models, emphasizing the importance of data-driven improvements and systematic studies to ensure the reliability of GENIE for experimental neutrino physics.GENIE is a new neutrino event generator designed for experimental neutrino physics, aiming to develop a 'canonical' Monte Carlo generator that can simulate neutrino interactions with all nuclear targets and flavors from MeV to PeV energy scales. The focus is on the few-GeV energy range, which is crucial for long-baseline precision neutrino experiments using accelerator beams. GENIE is a large-scale software system, consisting of approximately 120,000 lines of C++ code, and has been widely adopted by neutrino experiments. It addresses unique challenges in neutrino simulations, such as the transition between non-perturbative and perturbative regimes, and supports the full life-cycle of simulation and analysis tasks. The article discusses the paradigm shift brought by GENIE in neutrino physics simulations, highlighting the challenges in simulating neutrino interactions over a broad energy range and the importance of accurate models for various physical processes. It also provides an overview of the physics models available in GENIE, including nuclear physics models, cross section models, and hadronization models. The article describes the object-oriented design of GENIE, its applications and utilities, and the structure of the GENIE collaboration. Additionally, it covers the tuning and validation of the physics models, emphasizing the importance of data-driven improvements and systematic studies to ensure the reliability of GENIE for experimental neutrino physics.