Enterprise Resource Planning (ERP) systems have established a reputation in the world of business as indispensables tools that integrate all departments and functions across a company into a single computer system. However, implementing an ERP system does not always result in enhanced organizational performance. In order to ensure successful implementation, companies should study the critical factors having an impact on the whole procedure. In this context, the present study proceeds in developing and testing an original conceptual framework (research model), which explores the factors having an impact on ERP implementation success (internal environment, technology-related issues, implementation team, end-users), as well as the impact of the implementation itself on organisational performance. The proposed conceptual framework was tested, using a newly-developed structured questionnaire, in a sample of 204 Greek companies that have already implemented an ERP system. The Structural Equation Modelling (SEM) technique was used in order to test the research hypotheses.