Enterprise Resource Planning (ERP) systems are nowadays adapted to the company-specific requirements by customization. Whereas smaller and general feature changes are performed quickly by parameterization, changes of a company's business activities may lead to enormous changes in the system's code. To overcome this problem, we introduce an approach of a flexible ERP system which can be adapted by specific business models for each individual company/tenant by using model engineering techniques. Our approach REAlist is built on top of a generic data structure based on the Resource Event Agent (REA) business modeling ontology and provided as a cloud based service. Business models for each company are described by a domain specific language called the REA-DSL. REAlist enables companies to incorporate business activity changes quickly and without extensive code changes.