Increasingly, embedded devices are being equipped with ARM processors. Because of ease-of-use and widespread support for drivers and applications, Linux is often used as the OS of choice although it consumes a significant amount of the device’s limited resources. In this paper we propose Unikraft, a fully micro-library operating system and build tool which allows for building specialized OSes and software stacks targeting one application, while removing unneeded functionality. As a proof of concept, we port Unikraft to the Raspberry Pi 3 B+ and to a Xilinx Ultra96-V2. On these boards, Unikraft is able to boot in 88-158 milliseconds, consume only hundreds of KBs of memory when running real-world application such as NGINX while still providing visible reductions in power consumption compared to Linux distributions. Unikraft is an open source project.