The aim of this document is to describe the requirements for the ATLAS High Level Trigger Selection Software. In order to do this, the scope and context of the software project are first described, and an overview of the software is given. The software is logically divided into areas for which requirements, constraints and use cases are listed. They are also been given for the interfaces shared with external software. Requirements are prioritised as either high, medium or low. The priorities indicate how soon these requirements should be met by the software; it is expected that eventually all requirements will be met.