Pimsleur is a series that produces audio-only language learning courses. Originally created by Paul Pimsleur it is now produced by Simon & Schuster. Typically there are three levels with 30 units each. Some have a plus level with 10 further lessons Others are yet to be released. The programs are built so that the recordings prompt you to recall a fact just before you forget it, thus strengthening your memory of that language fact (vocab or grammar construction.) See also: Wikipedia: Pimsleur_language_learning_system