Device descriptions play an important role in the design and commissioning of modern automation systems and help reducing the design time and costs. However, all established device descriptions are specialized for certain purposes and suffer from several weaknesses. This hinders a further design automation, which is strongly needed for the more and more complex automation systems. To overcome these problems, this paper presents novel Ontology-based Device Descriptions (ODDs) along with a layered ontology architecture and triple store based database backend. It enables a formal, unified and extensible specification of devices, ensures their comparability and facilitates a computer enabled retrieval, selection and interoperability evaluation. The scalability of the approach to several ten thousand devices is demonstrated.