Atom interferometers exploit the quantum mechanical, wavelike nature of massive particles to make a broad range of highly precise measurements.  Recent technological advances have opened a path for atom interferometers to contribute to two areas at the forefront of modern physics: gravitational wave astronomy and the search for dark matter.  In this talk, I will describe a new experiment, MAGIS-100, that will use a 100-meter-tall atom interferometer to pursue these directions.  MAGIS-100 will serve as a prototype gravitational wave detector in the mid-band frequency range 0.1 Hz to 10 Hz, which is complementary to the frequency bands addressed by laser interferometers such as LIGO and the planned LISA experiment.  I will discuss the scientific motivation for gravitational wave detection in the mid-band.  In addition, I will explain how MAGIS-100 can look for ultralight dark matter, a well-motivated class of dark matter candidates that behave as coherently oscillating fields.  Finally, I will briefly introduce a related atom interferometry experiment we are building at Northwestern to perform fundamental tests of gravity.