With the evolution of Information Technology, especially the Internet, system integration is becoming a common way to expand IT systems within and beyond an enterprise network. Although system integration is becoming more and more common within large organizations, however, the literature review had found IS research in this area had not been sufficient, especially for the development of integration solutions within large organizations. It has made research like this one conducted within a large NZ government agency necessary. Four system integration projects were selected and studied using case study research methodology. The case study was designed and conducted using guidelines mainly from the well-known R. K. Yin’s (2002) “Case Study Research” book. The research was set to seek answers for a series of research questions, which were related to requirements of system integration and challenges for solution development. Special attention had been given to XML applications, as system integration and XML were found to be coupled in many system integrations and frameworks during the literature review. Data were first gathered from all four projects one by one, and then the bulk of analysis was done on the summarized data. Various analysis methods including chain-of-evidence, root-cause-analysis and pattern-matching were adopted. The principles of interpretive research proposed by Klein and Myers (1999) and triangulation were observed. In conclusions, a set of models have been derived from the research, namely a model for clarifying integration requirements; a model for integration solution architecture; a model for integration development life cycle and a model of critical success factor for integration projects. A development framework for small to medium size integration projects has also been proposed based on the models. The research also found XML application indeed would play an important role for system integration; the critical success factors for XML application included suitable development tools, development skills and methodologies.