The Modular C++ Build System is a set of makefiles, scripts, and policies for managing, testing, and building large C++ projects. The fundamental idea is that we divide the large project into several smaller pieces called subprojects. Subprojects are both a physical concept (ie. how the files are named and organized) as well as a logical concept (ie. how the classes and functions are named and organized). Each subproject encapsulates a moderate amount of related functionality; subprojects usually contain tens of source files and include Makefile/Autoconf fragments. Toplevel scripts automatically integrate these fragments with a great deal of pre-defined functionality to create non-recursive configure and Makefile scripts. The build system is designed so that subprojects can be easily moved between projects. The build system includes support for dependency tracking and a subproject called utst which provides a framework for writing unit tests. The mcppbs user guide and utst user guide include tutorials and reference documentation.