MaRTE OS (Minimal Real-Time Operating System for Embedded Applications) is a real-time kernel for embedded applications that follows the Minimal Real-Time POSIX.13 subset, providing both the C and Ada language POSIX interfaces. It allows cross-development of Ada and C real-time applications. Mixed Ada-C applications can also be developed, with a globally consistent scheduling of Ada tasks and C threads. Details on the architecture and implementation of the kernel are described, together with some performance metrics.