In today.s world, from TV sets to washing machines or cars, almost every electronic device is controlled by an embedded system. These systems are handling many tasks simultaneously. By using an operating system, handling of different tasks simultaneously is done in a more standardized fashion. The purpose of this thesis is to design and write a portable real-time operating system for embedded systems, which can be compiled with any application by using an ANSI C compiler. The main target is to design it as small as possible to fit the smallest microcontrollers. Other targets are high flexibility, optimal modularity, high readability and maintainability of the source code.