A hypermedia application offers its users a lot of freedom to navigate through a large hyperspace, described by a domain model. Adaptive hypermedia systems (AHS) aim at overcoming possible navigation and comprehension problems by providing adaptive navigation support and adaptive content. The adaptation is based on a user model that represents relevant aspects about the user. In this paper, we concentrate on the adaptation engine (AE) that is responsible for performing the adaptation according to the adaptation rules specified in the adaptation model. We analyze the dependencies between the authoring process and the functionality of the adaptation engine. From this we conclude how the authoring process can be simplified by a more powerful AE. In particular, a well-designed AE should be general purpose (i.e., not application domain specific) and should guarantee that the interpretation of the rules is deterministic, always terminates and produces the results desired by the author.