The early recognition of the patient whose clinical condition is deteriorating is a key patient safety strategy, enabling timely clinical intervention to prevent serious adverse incidents for patients [1–۴]. Longitudinal patient monitoring systems, for example, the Early Warning Scores (EWS) (NEWS in the UK and Ireland) and the Maternity Early Warning System (MEWS), are recommended to detect the deteriorating patient in many countries [5–۹] despite conflicting evidence as to their success at improving patient outcomes [10–۱۳]. Challenges to the successful implementation and evaluation of EWS include failure to heed the socio-cultural and organisational context [14, 15] and implementation in a ‘piecemeal’ manner without acknowledging the complexity of such an intervention [16]. This study is concerned with the implementation evaluation of a longitudinal patient monitoring system specifically designed for adult patients in the unique environment of the Emergency Department (ED) setting. This system is known as ED-ACE where ACE stands for Adult Clinical Escalation. A participatory action research (PAR) approach is taken where socio-technical systems (STS) theory and analysis informs the implementation and evaluation through the improvement methodology of multiple sequential ‘Plan Do Study Act’ (PDSA) cycles. Lack of understanding of the complexity of forces acting both within and on healthcare systems has led to many failures in attempting to improve patient outcomes [17, 18]. The many potential advantages to healthcare of applying STS theory described by Carayon et al. ([19] p.3) as “adopting a systems approach aimed at identifying multiple system elements, their interactions and their impact on the quality of care, as well as understanding the key adaptive role of people in the system” have been highlighted [19– ۲۷]. The term STS was coined by Trist and colleagues in the Tavistock Institute in London in the 1950s and later taken up by Klein to recognise the interaction between technical and social factors in organisations [28–۳۰]. When trying to change a system STS would stress the need to consider the technical and social factors and the impact of the change on other aspects of the system [31, 32]. This study applies STS theory and analysis for the first time to the implementation and evaluation of an ED longitudinal patient monitoring system. STS analysis will be conducted to describe and understand the ED environment and to inform the implementation and evaluation of EDACE. This analysis will take place at three levels: process functionality; communication, information and knowledge flow; and the social system (social relations, team, trust and accountability) using an STS analysis framework that has been developed in aviation safety research [33–۳۵]. STS allows us to analyse the transformation of information into knowledge and the sharing of that knowledge and therefore can be applied to systems where there is currently a reliance on paper-based charts and whiteboards as in many EDs [36]. The ED-ACE being implemented and evaluated in this study is paper-based.