This research investigates why certain enterprise resource planning (ERP) system adopters have pursued high levels of software customization during implementation despite the generally accepted best-practice heuristic of limiting customization. Qualitative data from ERP adoption projects and consultants working with ERP implementations have been collected. This study empirically identifies customization drivers and explains their relationship to customization. The results suggest that high customization may occur because of: unnecessary redevelopment of functionality that is available in the ERP system standard, resistance to change based on cultural issues and low project acceptance, insufficient weight given to the implementation team's recommendations, and the implementation team's lack of opposition to customization requests. The results of this study also explain how these problems occur and why they lead to overcustomization.