The CernVM File System is a client-server file system developed to deliver software stacks onto virtual machines in a fast, scalable, and reliable way. CernVM-FS is implemented as a FUSE module. It makes a specially prepared directory tree stored on a web server look like a local read-only file system on the virtual machine. CernVM-FS uses outgoing HTTP connections only, thereby it avoids most of the firewall issues of other network file systems. It transfers data file by file on demand, verifying the content by SHA-1 keys.