Applications based on plug-in architectures are extensible through thirdparties and enable customized user environments. We argue that extensibility and customization are important features in enterprise application software. In an ongoing research project we are developing a plug-in platform in .NET for the enterprise domain. Targeting the enterprise domain raises special requirements with security, reliability and versioning. This paper presents models for host and plug-in integration that address execution of plug-ins in reliable settings and allowing independent evolution of core applications and plug-ins and it will show how this has been solved in a consistent and transparent way in the .NET plug-in framework.