Environmental assessment (EA) is intended to ensure that environmental issues, in a broad sense, are considered in decision making. EA is globally institutionalised through national and international legislation, policies and guidelines, and a field of practice. However, there is a gap between the expectations on EA presented in these regulatory and guiding documents and how it is addressed in practice. This thesis explores the reasons for this gap, focusing on EA practitioners’ daily work. The aims of the research are development of theory and generation of new empirical knowledge about how EA practitioners think and act in their daily practice. At the heart of the research is the development of a conceptual framework, space for action, that centres on practitioners’ possibilities for influencing practice. The two-dimensional framework is developed iteratively, through empirical and theoretical investigations. The empirical investigations centre on: challenges for practitioners from the multiple perspectives available to apply when deciding appropriate actions; how practitioners think and act when seeking possibilities to influence practice; and, the consultant’s role in determining quality. The theoretical basis for the framework integrates and builds on earlier work by planning theorists and elements from frame theory. The first dimension in the framework concerns how practitioners decide on appropriate actions. This process restricts which perspectives practitioners decide to act on and argue for in practice and, subsequently, which perspectives that have potential to be addressed in EA. The second dimension concerns whether these suggestions for appropriate action are enacted and agreed upon in EA processes. These interactions restrict practitioners’ possibilities of exerting influence on practice, in terms of both the actual scope of an EA, and how the issues and impacts included are addressed, hence influencing EA quality. The results reveal that these dimensions are important for understanding the gap between expectations of improvements and actual practice. They also reveal a need to recognise the evolving and multi-perspectival character of EA, together with opportunities for advancing the field of practice through critical reflection, reframing and multi-profession collaboration. Overall, this thesis contributes to understanding the important role of practitioners in shaping the field of practice, and provides a new theorisation that strengthens the practitioner focus in EA research.