As with any diagnosis, the underlying purpose of a multimorbidity' one is to identify and establish the impact of a person's health conditions on their lives and to facilitate personalized decisions regarding proposed interventions. Clinicians routinely make decisions about the use of interventions for people with multiple long-term conditions. This is challenging because evidence to support this process currently relies on guidance on single health conditions for people without multimorbidity, typically taking fewer medications. Establishing the person's preferences over relevant criteria is central to a person-centered decision-making process, and it is particularly challenging, given the complexities of the person's multiple conditions. The final challenge is in combining the clinician's best estimates of the benefits and harms of possible interventions with the person's preferences. A review of these challenges, drawing on the NICE guidelines, leads to a proposal for using a Multi-Criteria Decision Analysis-based support tool for personalized shared decision making for multiple long-term conditions.