This dissertation explores the relationship between standardisation and discretion in professional work at street level, using the priority setting of triage nurses as its case. Triage nurses are employed at the frontline of emergency medical services, where they work to assess the urgency of patientsâ complaints. This work can be very challenging, requiring rapid assessments of a large group of unknown and unsorted patients, some of whom may be critically ill. To aid these assessments, emergency services have increasingly introduced standardised triage systems that specify how nurses should proceed in interpreting and prioritising cases. Triage systems reflects a broader trend in healthcare, which has seen a widespread introduction of clinical practice guidelines, all seeking to generate uniformity and quality control by streamlining clinical decision making. The introduction of these guidelines has been described as an unprecedented form of standardisation of professional clinical work, but there is little consensus regarding their effects. While proponents argue that clinical practice guidelines are an important means of improving quality and efficiency, critics denounce them for promoting bureaucratisation, homogenisation and so-called âcookbook medicineâ. Observing this impasse, there have been calls for an empirically grounded âsociology of standardisationâ to acknowledge that guidelines can have different effects in different settings and to explore standardisation on a case-by-case basis. Informed by that proposal, this dissertation explores the relationship between standardisation and discretion in triage nursesâ priority setting. The dissertation is based on nine months of fieldwork in a Norwegian emergency primary care clinic (EPCC), where nurses were required to assess patients using the Manchester Triage System (MTS). Observations revealed that nurses regularly departed from the MTS while also seeming to be influenced by the system in a number of ways. On this basis, the dissertation addresses the question of how and why nurses departed from the MTS and, conversely, how the MTS influenced their assessments. The introduction and the four associated articles show how nurses supplemented the MTS with additional skills and knowledge, and how this led them to adjust or override the priorities formally prescribed by the system. While they had several reasons for so doing, their primary concern was to âcorrectâ the MTS and to ensure more precise prioritisation of patients. However, the MTS also played a significant role in their assessments by restricting, enabling and supporting priority setting.Beyond shedding light on discretionary practices among triage nurses, the dissertation makes three more general contributions to the sociology of standardisation. First, it helps to bridge the gaps between the sociology of standardisation and the literatures on street-level bureaucracy and categorisation. In so doing, it identifies fruitful theoretical linkages for future studies of standardisation and discretion in street-level categorisation of clients.Secondly, the in-depth exploration of nursesâ use of the MTS provides a rich account of the difficulties of streamlining clinical practice. Despite its elaborate design, the MTS was too âthinâ to match the complexity of triage nursesâ work, and to follow it unreflectively would be to the detriment of both patients and staff. For that reason, nurses found it necessary to render the guidelines âthickerâ by making situated judgments, illustrating the crucial role of additional skills and knowledge in making standards work.Finally, the dissertation shows how the MTS (despite its shortcomings) affected nursesâ work in multiple ways, illustrating how guidelines interact with professional practice. In so doing, the dissertation transcends the either/or language that characterises much of the debate around standardisation, instead providing a nuanced account of the interplay between prescribed and discretionary aspects of triage nursing.