The self-care management of chronic disease patients is complicated by various everyday decisions that range from routine ill-structured problems, e.g., “What to eat?” to uncertain symptoms-related decisions, e.g., “Why do I feel tired?” Such decisions can have significant consequences on a patient’s health, treatment, care, and associated medical costs. Due to the complexity involved in understanding and analysing everyday decision making, there is a lack of empirical research to guide the development of self-care decision aids. This thesis aims to address this problem by formulating and illustrating the Critical Illness Self-care Decision Aid (CISDA) process through a coherent, structured, integrated design and development process using a case study. Following a literature review, the problems in current approaches and the criteria needed for the development were derived from evidence-based frameworks such as chronic disease management, decision aids standards and complex interventions development process for future designs. Mixed methods were used including: focus groups, interviews, questionnaire, Cognitive Work Analysis and case scenarios for not only constructing an account of self-care needs and decisions but also to evaluate the development process and the decision support provided involving patients, doctors, caregivers, non-medical experts like psychologists and IT/Systems engineers. The CISDA process consists of: (i) needs assessment, (ii) theory formation, (iii) modelling, (iv) integration, (v) interface design and development, and (vi) evaluation for addressing the relevant intersection of human factors, systems engineering, and software engineering. This thesis should prove useful to not only systems engineers but also to a range of practitioners concerned about decision making, maintaining a user's cognitive perspective during specification and analysis of a complex system.