Complex information systems may be viewed as systems that cut across functional boundaries within an organization and even organizational boundaries. These include enterprise resource planning (ERP) systems, supply chain management (SCM) systems, customer relationship management (CRM) systems, product lifecycle management (PLM) systems and business-to-business (B2B) systems. Such systems pose significant knowledge barriers for assimilation, require coordination with internal and external actors, and entail reengineering of both cross-functional and inter-organizational business processes. Moreover, organizations progress through various stages of assimilation such as initiation, experimentation, implementation, and routinization in assimilating complex systems.An often-overlooked consideration when dealing with such systems is that organizations may not completely assimilate them and even abandon them midway through the assimilation process. Such stories are well-documented in the popular press (e.g., failed projects, cancelled contracts) but generally do not provide insightful explanations of the accompanying assimilation process. However, there is not much evidence in prior empirical literature as to how assimilation processes came together in real-world organizations or the differences in the assimilation processes between organizations that succeeded or failed when dealing with complex information systems.Conceptualizing assimilation as a process by which organizations move from the initiation through the routinization stages, this research strives to uncover facilitators that enable an organization to move to the next stage and inhibitors that may force organizations to stay in the current stage or completely abandon the assimilation process. Employing a multiple case-study approach involving both successful and failed projects of different complex systems with data provided by key informants, this research aims to uncover usable knowledge for researchers and practitioners.