XtreemOS system provides innovative solutions for several issues related to grid and cloud com-
puting and allow users to:
• Use the Grid like their Linux workstation,
• Run interactive and legacy applications on a Grid using POSIX and SAGA,
• Access their data transparently across the Internet with XtreemFS,
• Automatically discover computing resources matching their needs,
• Harness cluster power using Kerrighed SSI technology.

Our new Grid operating system introduces the following new technical challenges:
• Scalability: supporting hundreds of thousands of nodes and millions of users dynamically joining and leaving the Grid,
• Transparency: hiding the complexity of the Grid by distributed operating system services allowing to run new and legacy applications seamlessly,
• Interoperability: complying with all major standards such as POSIX and SAGA,
• Dependability: providing reliability and high availability through checkpointing and replication,
• Security: ensuring trust and integrity according to customizable policies.

Users logged into an XtreemOS box transparently exploit VO-managed resources through the standard POSIX interface. In contrast to middleware approaches, XtreemOS is an operating system able to execute any kind of application on the Grid, including unmodified existing applications as well as interactive applications. The XtreemOS software has been developed from scratch during a research project financed by the European Commission (FP6-IST-033576). XtreemOS is running on a wide range of hardware ranging from smartphones, PCs and Linux clusters. The PC flavor of XtreemOS runs on an unmodified Linux kernel. The cluster flavor is built upon the kerrighed technology .