This is an original theory of simultaneous interpretation (SI) based on relevant principles of cognitive psychology, psycholinguistics, and introspection by advanced professional practitioners about the mental processes and techniques utilized during the actual performance of the craft. The objectives of the theory are: 1) to describe the different tasks involved in the process of SI; 2) to isolate each task in order to target the corresponding skill during focused training; 3) to describe the flow of tasks into a seamless SI process and the correct techniques that help to optimize performance; and 4) to lay the grounds for devising training methods and skills-building exercises for advanced SI performance. Thus far existing SI skills-building exercises, including shadowing, dual tasking, and paraphrasing, tend to focus exclusively on intra-lingual tasks, without actual inter-lingual translation. As such they are designed for the beginner. In contrast, the present cognitive theory of SI focuses on advanced techniques and optimization training methods.