Overview: Astrophysics Telescope for Large Area Spectroscopy (ATLAS) Probe is a concept for a NASA probe-class space mission, the spectroscopic follow-up to WFIRST. ATLAS will multiplex its scientific return by obtaining deep slit spectroscopy with R=1000 over the wavelength range of 1-4 microns for ~200M galaxies imaged by WFIRST out to z = 7 in three tied surveys (Deep, Medium, and Wide), and deliver spectra that enable a wide range of diagnostic studies of the physical properties of galaxies over most of cosmic history. ATLAS is a 1.5m telescope with a field of view of 0.4 sq deg, and uses Digital Micro-mirror Devices (DMDs) as slit selectors, which enable a spectroscopic multiplex factor greater than 5,000. ATLAS will orbit around the Sun-Earth L2 Lagrange point. It will have a 5 year prime mission that includes Guest Observing time. ATLAS is designed to fit within the NASA probe-class mission cost envelope; it has a single instrument, a telescope aperture that allows for a lighter launch vehicle, and mature technology (DMDs can reach TRL 6 within 2 years).