The overall objective of the XtreemOS project is the design, implementation, evaluation and distribution of an open source grid operating system (named XtreemOS) with native support for virtual organizations (VO) and capable of running on a wide range of underlying platforms, from clusters to mobiles. The approach we propose is to investigate the design of a grid OS, XtreemOS, based on the Linux existing general purpose OS. A set of system services, extending those found in the traditional Linux, will provide users with all the grid capabilities associated with current grid middleware, but fully integrated into the OS. The underlying Linux will be extended as needed to support VOs spanning across many machines and to provide appropriate interfaces to the Grid OS services. Installed on each participating machine, the XtreemOS system will provide for the grid what an operating system offers for a single computer: abstraction from the hardware, and secure resource sharing between different users. It would thus considerably ease the work of users belonging to VOs by giving them (as far as possible) the illusion of using a traditional computer, and releasing them from dealing with the complex resource management issues of a grid environment. By integrating grid capabilities into the kernel, XtreemOS will also provide a robust, secure and easy to manage infrastructure for system administrators