After most large industrial companies have installed enterprise systems (ES), the managerial concern moves to the effective diffusion and infusion of ES throughout a company. Diffusion means the degree in which a system has been shared throughout the company and infusion is the degree in which a system has penetrated a company in terms of importance, impact, and significance. Little of the ES literature has considered these new larger issues. This thesis develops a model of internal diffusion and infusion of ES. The ES diffusion model presents the ES implementation as an iterative, cumulative, and virtuous process over time. The model recounts and validates the authentic characteristics of ES projects, places "use" as both product and fomenter of the implementation activity, and putting emphasis upon the importance of experiential learning. The ES infusion model develops a framework of ES capabilities to analyse the extent and quality of the use of ES in organizational contexts (i.e. infusion). To investigate these issues this research has borrowed the perspective of organizational introduction of a technological innovation from the innovation discipline. The scope of the investigation is limited to three selected organizations overseas, which had each already installed at least a single functional component of the system and had decided (or was working on) the diffusion of the system in further departments or locations. The data collection was a combination of retrospective and real-time analysis. Hence, a multiple-case studies for constructing theory from the observed field data was developed.