PRIMES-USA is a free, year-long research program in mathematics open to high school juniors and sophomores (or home schooled students of the same age) residing in the United States outside the Greater Boston area. Program participants work on exciting unsolved problems in mathematics. Applied mathematics projects include problems in theoretical computer science and computational biology. Juniors work on individual or group projects; sophomores work on group projects. Projects are mentored by graduate students and faculty from MIT and other universities via teleconferencing, using software and hardware tools for online collaboration, or by local mentors chosen by PRIMES.