||Comneon, as the software branch of InfineonTechnologies, provides APOXI for development ofmobile system software solutions. APOXI is a fullfledged C++-implemented application framework which provides rich functionality and a defined extension, adaptation, and customization scheme to realize mobile software solutions. Based on APOXI, mobile phone manufacturers realize their software solutions for mobile handsets in a multi-stage adaptation and customization process.Due to complexity and diversity of today’s mobilesoftware solutions, an increased demand for an easier customization process has been observed which led to the development of the Aragon tool. Aragon is an Eclipse based interactive and graphical GUI builder tool for customization of man machine interfaces. This paper describes requirements, design, and architecture of theAragon tool. Starting with an overview of the mobilesystem domain and development process and, from that, deriving requirements for a builder tool, the paper outlines the main design decisions taken and the resulting architecture of the Aragon tool, which distinguishes it significantly from comparable systems.