Enterprise Resource Planning (ERP) software packages play an important role in business today, while their implementation projects exhibit a high level of complexity and failure, hence the emergence of the line of research on their critical success factors (CSFs). Employing a vendor to implement ERP is the norm in most companies, and CSFs related to the vendor are cited in the literature; however, some studies suggest the in-house implementation alternative. This dissertation discusses the effect of using an in-house implementation on the implementation success through comparing such implementation to a vendor-led one taking place at one same company in the Middle East - and hence in the same settings - but at an earlier stage. At a secondary level, the dissertation studies the ‘project management’ and ‘top management support’ CSFs in both implementation stages and their effect on success. The dissertation employs descriptive quantitative analysis through a questionnaire directed to several categories of employees and managers besides the qualitative analysis through interviews with senior management.