Line Emission Mapper (LEM) is an X-ray Probe Mission concept to study galaxy evolution, directly addressing Astro2020 Decadal's Priority Area of Unveiling the Drivers of Galaxy Growth. LEM is optimized to explore the X-ray emission from the warm, extremely dilute gas residing within the galactic virial halos and beyond them, known as the Circumgalactic Medium (CGM), Intragalactic Medium (IGM)/Warm-hot Intergalactic Medium (WHIM), and resolve this emission from the bright Milky Way halo emission. A silicon-shell mirror and a microcalorimeter array with high spectral resolution sensitive in the soft X-ray band, covering a large field of view, will be able to map the CGM of a nearby galaxy in one pointing, which is beyond the reach of any of the planned missions.