The MPFR library is used by Macaulay2 for its arbitrary precision real number arithmetic and associated transcendental functions. It is available at http://www.mpfr.org/. The library is remarkable for the care taken to return correctly rounded results. It is hoped that this will form a good base for experimentation with algebraic algorithms that mix symbolic and numeric techniques. It is licensed under the GNU Lesser General Public License, version 3 or later.