Enterprise Content Management (ECM) systems are mostly implemented in organizations by acquiring commercial software packages and customizing them to meet the organizational requirements. The customization aspect of ECM systems lacks empirical research. This paper explores the concepts of ECM customization and issues identified with ECM customization. The data are based on an in-depth case study from the oil industry and complemented with a secondary analysis of 60 vendor-reported cases of ECM implementations. The results show considerable customization challenges related to ECM, especially concerning integration, usability and functional adaptation. A resulting framework of customization concepts in ECM is suggested and discussed, along with issues for further research.