The STX B+ Tree package is a set of C++ template classes implementing a B+ tree key/data container in main memory. The classes are designed as drop-in replacements of the STL containers set, map, multiset and multimap and follow their interfaces very closely. By packing multiple value pairs into each node of the tree the B+ tree reduces heap fragmentation and utilizes cache-line effects better than the standard red-black binary tree. The tree algorithms are based on the implementation in Cormen, Leiserson and Rivest s Introduction into Algorithms, Jan Jannink s paper and other algorithm resources. The classes contain extensive assertion and verification mechanisms to ensure the implementation s correctness by testing the tree invariants. To illustrate the B+ tree s structure a wxWidgets demo program is included in the source package.