Enterprise Resource Planning (ERP) Systems are comprehensive off-the-shelf packages that have to be configured to suit the requirements of particular organisations. Implementation tools that guide the project team through the configuration process support the system individualisation. A key component of configuration is reference models that describe the functionality and structure of the ERP system. This research in progress paper discusses the shortcomings of current ERP reference models. Proposed modifications to reference models include support for specific model configurations, the interdependence of these configuration decisions on other reference models and the subsequent consequences of these decisions. Reference models, once configured in this way, become enterprise-specific. In addition, configuration decisions need to be documented and used throughout the ERP system lifecycle. In order to facilitate understanding and explanation of reference model modifications, scenarios and design rationale are used to augment the reference models. A program of further research including tool support and empirical studies is presented.