MG4J (Managing Gigabytes for Java) is a free full-text search engine for large document collections written in Java. The starting point for understanding MG4J is a look at the overview of the Javadoc documentation, and at the manual. Then, the Javadoc class documentation can provide more insights. Please look at the overview section for package dependencies. We know that the documentation is not complete, and sometimes difficult to locate. Please report any major difficulties in understanding a part of MG4J, so we can build better docs! MG4J is free software distributed under the GNU Lesser General Public License. Sebastiano Vigna (vigna@acm.org) Paolo Boldi (boldi@di.unimi.it)