The physics motivation for long-lived particle (LLP) searches is very strong, both from the bottom-up and top-down point of view. They can be the smoking gun signature for many beyond-the-Standard-Model (BSM) scenarios, including those which solve the Hierarchy Problem, Dark Matter, Baryogenesis or other fundamental mysteries. On the other hand, while the LLP search program at the LHC is making rapid progress, many gaps in coverage remain, and large classes of scenarios will be very difficult to probe even in the future. I will explain how MATHUSLA can close those gaps in coverage to extend sensitivities to LLPs with lifetimes near the BBN limit of 0.1 seconds. There has been significant recent progress on the theoretical front, with a large white paper in progress to make the comprehensive physics case for this detector, and analyses which demonstrate that even a simple design can extract significant information from the LLP decay. On the experimental front, a prototype detector is currently under construction at CERN and will be online later this year. There are also significant opportunities for using MATHUSLA for cosmic ray physics.