MAGIS-100 is a next-generation instrument that uses light-pulse atom interferometry to search for physics beyond the standard model, to be built and installed at Fermilab. We propose to search for dark matter and new forces, and to test quantum mechanics at new distance scales. The detector will use the existing 100 m vertical NuMI access shaft to make it the world's longest baseline atom interferometer. To maximize the sensitivity of the experiment, we will use the latest advances in atomic clock technologies. The experiment will be a significant step towards developing a 1000 m baseline detector, with sufficient sensitivity to detect gravitational waves in the `mid-band' from 0.1 Hz - 10 Hz, between Advanced LIGO and LISA experiments. Here we describe an overview of the experiment and its physics reach.