The aim of this 'download' is to present the basics of mental disorders. The target population is medical students, but general public readers may also find it useful. The mental disorders form a huge, mysterious and problematic body of knowledge. They also indicate a huge body of ignorance. The mental disorders represent a major challenge to contemporary science, government and humanity. When the less severe forms are included, more than 25% of the people in western populations will experience a mental disorder at some time in their lives.