PDE.Mart is a network-based problem-solving environment (PSE) for solving partial differential equations (PDEs) in numerical simulations and academic research, as well as in educational settings. The client-server protocol consists of a Web-browser-enabled graphical user interface, PDE-GUI, that runs on client machines to manage the server connection, geometric and model specifications, computational method selection, and postprocessing; a server system, PDE-Server, to build computational engines and provide PDE solution services on the host machine; and a library, PDE-LIB, that contains building blocks for developing network-based and PDE-oriented PSEs.