Middleware in automation serves the connection of hardware and software components of applications in automation-like control or human-machine interfaces. To cope with today's flexibility requirements, there is an urgent demand to not only communicate data but to describe the semantics of the distinct elements of the distributed information space. Therefore, middleware in automation needs to provide some means for information modeling. To derive general concepts and recommendations on how to implement information models on middleware in automation, we discuss several relevant standards that provide partial information models. We then identify the fundamental information modeling constructs of these partial models and derive general requirements for the information modeling capabilities of middleware in automation.