This white paper provides an introduction to and overview of seL4. We explain what seL4 is (and is not) and explore its defining features. We explain what makes seL4 uniquely qualified as the operating-system kernel of choice for security- and safety- critical systems, and generally embedded and cyber-physical systems. In particular, we explain seL4’s assurance story, its security- and safety-relevant features, and its benchmark-setting performance. We also discuss typical usage scenarios, including incremental cyber retrofit of legacy systems.