This thesis constitutes an empirical study of accounting in action, focussing attention on patient based cost systems. The thesis contributes an in depth understanding of the mobilisation of casemix and related information systems at a large regional hospital, Health Waikato (HW), in the centre of the North Island of New Zealand. The field research consisted of primarily unstructured and semi-structured interviews and documentary analysis. I present the research in the later part of the thesis from a constructionist, interpretive perspective. This consists of richly descriptive case studies of aspects of the change process as it has impacted upon the research site. The themes of the analysis are related, at the macro level, to the resurrection of neoclassical economics policies and the relative ascendancy of free market solutions. The process through which areas of knowledge and in this case particularly public policy become problematised is explicated. My research attempts to describe the experiences and perceptions of medical and managerial\financial staff at a work unit level within a single hospital. A part of this process has involved investigation of the implementation of traditional accounting technologies in unfamiliar environments. The research is primarily concerned to elaborate upon the social context of accounting systems implementation using theoretical insights derived from Latour (particularly: 1987, 1993). The research has sought to explicate the change process as a process of translation. Traditional accounting techniques have been explicated as “black box” technology with which the organisation has been redefined in economic terms. In the study, the power of accounting in the translation and inscription of data (the fabrication of accounting systems per Preston et al, 1992), is central to understanding the role of accounting systems as technology. Drawing from the work of Latour helps to provide a frame of reference to allow an assimilation of disparate changes and influences as they have come to affect the health sector at a national level, within New Zealand, and also at an organisational level, within a large regional health provider. The research contributes in explicating the relevance of Latour’s rules of method, and underlying theoretical framework for an organisational analysis focusing upon accounting. Latour uses a very general conception of technology which encompasses anything emerging from what he terms the process of “translation”. In this context Latour uses the term to refer to the production or “fabrication” of “quasi-objects”. This is most easily seen as consisting of the physical objects which “populate our western societies”, but for Latour also includes inscriptions and “facts/artefacts”. I regard accounting and information systems as consisting of mixtures (or perhaps “collectives”) of technological quasi-objects in this very general sense. The focus of the research has been upon the identification of problems, the choice of accounting techniques and their implementation. Together with other devices the use of accounting techniques may be seen as a central part of the process through which change is made acceptable within the organisation. Supporters are enrolled into the change process in part by being exposed to the accounting inscriptions which are used to represent the cost and profit “reality” of their unit and the whole organisation. The research process has involved detailed investigation on a case by case basis to enable a thorough description of the accounting techniques being put in place. The title of the thesis is based on Latour (1987) "Science in Action". Conventions developed in Actor Network Theory might suggest my title would be better understood as "Accounting as Actant" but it seems to me that Latour was clearly aware of this same point when he chose this title for his book.