ZooLib allows one to write a single set of C++ sources which can be compiled into native executables for Mac OS, Windows, BeOS, or POSIX-compliant systems that use the X Window system (such as Linux). Zoolib provides a GUI toolkit with a uniquely flexible layout system. It also provides a single-file database format, TCP networking, and extensive debugging support. ZooLib applications are multithreaded. ZooLib requires only minimal support from the underlying OS and platform GUI layer, and thus could be ported to a completely new platform without too much difficulty. ZooLib is fully production quality on Windows and MacOS, completely implemented but untested on BeOS, and not yet complete on POSIX. Please note that the sources from the "demo" branch are also required to build ZooLib or to get started writing your own ZooLib applications.