In this book, we have focused on the research of content-based MIDI music retrieval and computer-aided composition based on musical whistling. For MIDI music retrieval, a prototype system called “Whistle-for-Music” is developed. This system enables users to retrieve MIDI format music by whistling a melodic fragment. It consists of three essential components which are query processing, MIDI processing and an approximate search engine. For query processing, we have achieved a real-time and robust whistle-to-MIDI converter. For MIDI processing, the extracted features from MIDI files contain individual, local and global melodic descriptions. In order to match a querying pattern with target.