This document is a manual for the library GRÖBNER, which is a C implementation, written by the first author, of the second author's Gröbner bases algorithm. The implementation is part of the first author's diploma thesis, [10] and is modeled after earlier implementations by the second author in various symbolic computation languages, notably in Mathematica, see [2]. The design reasons for using C in the present implementation are described in [3]. Summarizing, a good implementation of an algebraic algorithm should combine both speed and ``polymorphism''. By using C and a systematic renaming mechanism, which is based on a proposal by the second author and is described in the present document, we believe that, in fact, we can achieve C speed and reasonable polymorphic generality.