The IMO's forthcoming Life Cycle Assessment (LCA) legislation marks a shift toward greater scrutiny of greenhouse gas (GHG) emissions from both conventional and alternative marine fuels. LNG-fueled vessels have the potential to achieve lower life cycle GHG emissions in the near term compared to conventional fuels, but this relies heavily on effective management of engine-out methane slip. Recent research has demonstrated that a wide range of Tank to Wake (TtW) GHG emission factors can be achieved from the current fleet of LNG-fueled vessels depending on engine technology level and vessel duty-cycle. Unlike CO2 emissions, methane slip cannot be estimated from fuel consumption alone. Existing LNG vessels can provide a GHG and air quality benefit relative to conventional fuels, however operators must develop a clear understanding of the factors influencing GHG emissions on a vessel-by-vessel basis, adopt reliable methods to quantify these emissions, and proactively implement strategies to mitigate them. In this presentation, real-world emissions data and first-hand experience from controlled sea trials and monitoring of commercial operations of three LPDF LNG vessels spanning various engine technologies and powertrain configurations are discussed. Vessel 1 features a 2015 engine with generator drive, Vessel 2 employs a 2019 engine with hybrid-electric powertrain, and Vessel 3 uses a 2015 engine with direct mechanical drive. A wide range of TtW GHG emission factors influenced by engine load, vessel operation, and installed technologies illustrate the limitations of current GHG emission inventories in representing real-world LNG vessel emissions. In the second half of the presentation, outstanding challenges in the collection of reliable real-world emissions data are discussed. This includes the availability of appropriate sensor technologies, best practices for measurement protocols, and how real-world uncertainty impacts reporting of measurements. A robust, low-cost, calibration free methane slip sensor developed at UBC is presented, and its utility is demonstrated for CH4 emission monitoring on marine vessels. This presentation will provide the audience with a science-based overview of the current state-of-the-art in LNG vessel GHG emissions performance, how to effectively quantify them, and how to practically mitigate them from existing vessels.