NANOGrav is a collaboration of observational and theoretical astrophysicists from more than a dozen universities, colleges, and observatories. We include faculty, scientific staff, postdoctoral scholars, graduate students, and undergraduate students, and high school students. We seek to measure gravitational waves--ripples in space-time--by detecting their effect on radio wave signals traveling from pulsars--rapidly rotating neutron stars--to the Earth. Our measurements are most sensitive to gravitational waves with periods of a few years, which corresponds to frequencies of a few nanohertz--hence the "NANO" in our name, "NANOGrav." The strongest sources of such gravitational waves are likely to be binary systems in which two massive black holes orbit one another. Such binary systems form when galaxies merge with one another.