Background: Intensive care treatment can be life-saving but is invasive and distressing for patients receiving them, and not always successful. Deciding whether or not a patient will benefit from intensive care is a difficult clinical and ethical challenge. Objectives: To explore the decision-making process around referral and admission to ICU and develop an intervention to improve it. Methods: A mixed methods study including: a) Two systematic reviews investigating factors associated with decisions to admit to an intensive care unit (ICU) and experiences of clinicians, patients and families of the process. b) A focused ethnographic study including observation of decisions and interviews with ICU doctors, referring doctors, patients and families in six NHS Trusts in the UK Midlands. 55 decisions regarding 46 patients were observed. 101 doctors were interviewed. c) A Choice Experiment questionnaire survey with paired patient profiles developed using data from the ethnographic study and systematic reviews. UK ICU consultants and Critical Care Outreach Nurses were invited to participate. Participants’ preferences for ICU admission were analysed using a Random Utility Framework and preference heterogeneity using a latent class logit (LCL) model. d) Using the data from systematic reviews, ethnographic study and CE we developed a Decision Support Intervention (DSI) including an ethical framework for decision-making, guidance for referral and supporting referral and decision support forms and patient and family information leaflets. It was tested for implementation feasibility in three NHS Trusts purposively sampled for size of ICU. An eight week implementation period was followed by six weeks data collection. Quantitative and qualitative data were collected to evaluate form usage, intervention acceptability, barriers and facilitators to implementation, and impact on decisionmaking. e) A tool to evaluate the ethical quality of decision-making related to ICU admission was developed, based on assessment of patient records. The tool was tested for inter-rater and inter-site reliability in 120 patient records across the three sites in the implementation feasibility study. Results: Influences on decision-making identified in the systematic review and ethnographic study included age, presence of chronic illness, functional status, presence of DNACPR order, referring specialty, referrer seniority, and ICU bed availability. ICU doctors used a gestalt assessment of the patient in making decisions. The CE showed age was the most important factor in consultant and CCOR nurses preferences for admission. The ethnographic study illuminated the complexity of the decision-making process, the importance of inter-professional relationships, and good communication between teams and with patients and families. Doctors find it difficult to articulate and balance benefits and burdens of ICU treatment for a patient. There was low uptake of the DSI although doctors who used it noted that it improved articulation of reasons for decisions and communication with patients. Several challenges were identified in developing an evaluation tool.