Health technology assessment (HTA) is one of the major research enterprises of late modernity, reaching into fields of previously autonomous professional practice, and critically interrogating the organisation and delivery of health care. The 'evaluation' of new health technologies within the field of HTA is increasingly a normative political expectation, as discourses of 'evidence-based' practice run through health policy in the UK and elsewhere. Despite its importance in governing the direction of innovation in health care delivery, there are hardly any empirical studies of HTA in practice. In this paper, we draw on two ethnographic studies of telehealthcare implementation and evaluation in the UK to explore the practical conduct of HTA, and we focus specifically on the social organisation and conduct of randomised controlled trials of these new technologies. The paper examines how evaluation forms a mediating set of practices that make the embedding or normalisation of a new technology possible; and present a simple model of the social and technical contingencies within the evaluation process.