Version 2.2.1 of the Contiki operating system has just been released. The Contiki operating system is designed for wireless deeply embedded systems that have memory on the order of a few kilobytes and a power consumption on the order of a few milliwatt. Within these severe resource constraints, Contiki provides a low-power multi-hop TCP/IP stack, a command-line interface, a flash-based file system, and loadable modules. Release 2.2.1 improves network performance, reduces power consumption, and fixes a number of bugs from previous releases.