Methods of ERP (Enterprise Resource Planning) systems modelling are presented in the paper. The modelling is necessary to adapt and ERP system to a company in such a way, that it fully supports a management at the operational level, i.e. it supports the business processes in the best possible way. The choice of methods (business process and graph grammarbased)described in the paper is based on the authors’ experiences in several dozen of implementation projects, realized in large Polish enterprises. Themethods can be used to select the best ERP system for a company, to designits implementation, and to customize the system accordingly to the requirementsof a company.