XSSR: An experimental system for string rewriting -- decision problems, algorithms, and implementation. Here we present the eXperimental System for String Rewriting, XSSR, which is a special purpose computer algebra system for combinatorial group and semigroup theory. It is intended for effectively performing computations in monoids and groups that are given through finite presentations in terms of generators and defining relations. This system, which has been developed in a joint research project at the University of Kaiserslautern and the University of Kassel, is based on rewriting techniques, adopted to the special situation of string rewriting systems at hand.