In this presentation, we will describe the scientific opportunities that will arise from the Matter in Extreme Conditions Upgrade project at SLAC. The new experimental capabilities will access a novel physics regime where we expect to rapidly produce knowledge in the areas of High Energy Density Science and Inertial Fusion Energy. Presently, the MEC instrument is DOE’s flagship facility for discovery science in plasma physics and high energy density science that has produced over 100 publications with 25% of the papers in high-impact journals and letters; MEC research results have been highlighted in international TV and radio shows including the CNN, BBC, and DW news. DOE has recently approved a community-supported plan to upgrade the MEC instrument that will bring high-repetition rate petawatt lasers to the LCLS’s world-class X-ray beam to support the discovery physics and inertial fusion energy science’s missions. The high-repetition-rate experimental setting (10 Hz) of the MEC-Upgrade will provide a unique opportunity to advance the field. In preparation for the upgrade, we have developed high-repetition rate jet targes, explored advanced laser ion beam acceleration physics regimes, and delivered the capability of ultrafast X-ray probing of high-intensity laser target interactions.