As the Internet of Things (IoT) emerges, compact operating systems (OSs) are required on low-end devices to ease development and portability of IoT applications. RIOT is a prominent free and open source OS in this space. In this paper, we provide the first comprehensive overview of RIOT. We cover the key components of interest to potential developers and users: the kernel, hardware abstraction, and software modularity, both conceptually and in practice for various example configurations. We explain operational aspects like system boot-up, timers, power management, and the use of networking. Finally, the relevant APIs as exposed by the OS are discussed along with the larger ecosystem around RIOT, including development and open source community aspects.