This text is a user’s manual for the open source machine learning system Clus. Clus is a decision tree and rule learning system that works in the predictive clustering framework. While most decision tree learners induce classification or regression trees, Clus generalizes this approach by learning trees that are interpreted as cluster hierarchies. We call such trees predictive clustering trees or PCTs. Depending on the learning task at hand, different goal criteria are to be optimized while creating the clusters, and different heuristics will be suitable to achieve this.