There is much evidence that top management support is the single most important determinant of IS project success. This is especially the case in complex and large-scale IS projects. Surprisingly however, despite its crucial importance there is only limited knowledge of the types of behaviour associated with top management support and the reasons for low support levels. This research aims to address this gap by focussing on two questions: (1) Which types of behaviour constitute the top management support for strategic IS projects? and (2) Why do managers sometimes fail to provide this support? To answer these questions. we analyzed the top management support during enterprise system implementations in five different organizations. We conducted cross-case analyses to determine the range of behavioural approaches associated with this concept. Based on our research questions, we propose a taxonomy that includes six categories of supportive behaviour of top management. from which potential reasons can be derived to either support or non-support strategic IS projects.