The late 1960s witnessed an unprecedented interest in the environment. One of the intellectual characteristics of this period was the rise of ecocentrism, a form of ecological reasoning that challenged the domination of anthropocentric environmental thinking and practice. The thesis briefly reviews the evolution of ecological forms of reason, and then poses two questions. The first question asks: "What is ecological reason and how does the literature conceptualise it?" This leads to a theoretical analysis of the forms of ecological reason discernable in the literature, and results in a 'matrix of ecological reason'. The three primary forms of ecological reason are described as 'Technocentric Ecology', 'Discursive Ecology' and 'Eco-social Ecology'. They differ in respect to different dimensions of ecological reason, the forms of communication employed (drawing here on Habermas), and the level of commitment to anthropocentrism or ecocentrism. This 'matrix' highlights the contested nature of ecological reason in the literature, and demonstrates that there is, yet, no clear agreement on what it means, or should mean. The second question examines the ecological rationality of environmental practice. The 'matrix' is employed in three case studies of environmental decisions that take place under the New Zealand Resource Management Act (RMA), and investigates the forms of ecological reason expressed in these decision processes. The results of this analysis show that Eco-social Ecology barely registers in these case studies, while the other two forms of ecological reason. Technocentric Ecology and Discursive Ecology are both highly visible in the rationality of the RMA, but with two important qualifiers. First, although there is a commitment to Discursive Ecology on the part of many professionals, there is also much concern that this form of reason undermines quality environmental decisions. Thus, there is significant ambiguity as to the role of the community (an important dimension of Discursive Ecology). This leads to the second qualifier. There is an uneasy relationship between these two forms of reason, at both the theoretical and practice level. This tension underpins the competing visions of the RMA as a scientifically driven process and as a community process. This thesis argues that this tension does not provide for a secure marriage of these two visions.