The Naval Observatory Vector Astrometry Software (NOVAS) is a free source-code library for computing various commonly needed quantities in positional astronomy. NOVAS can supply, in one or two subroutine or function calls, the instantaneous celestial position of any star or planet in a variety of coordinate systems. NOVAS also provides access to all of the “building blocks” that go into such computations—single-purpose subroutines or functions for common astrometric algorithms, such as those for precession, nutation, aberration, parallax, etc. NOVAS calculations are accurate at the sub-milliarcsecond level. The NOVAS package is an easy-to-use facility that can be incorporated into data reduction programs, telescope control systems, and simulations. NOVAS is used in the production of the U.S. parts ofThe Astronomical Almanac.