Background For people living with dementia, the capacity to make important decisions about themselves diminishes as their condition advances. As a result, important decisions (affecting lifestyle, medical treatment and end of life) become the responsibility of someone else, as the surrogate decision-maker. This study investigated how surrogate decision-makers make important decisions on behalf of a person living with dementia. Methods Semi-structured interviews were conducted with 34 family members who had formally or informally taken on the role of surrogate decision-maker. Thematic analysis of interviews was undertaken, which involved identifying, analysing and reporting themes arising from the data. Results Analysis revealed three main themes associated with the process of surrogate decision-making in dementia: knowing the person's wishes; consulting with others and striking a balance. Most participants reported that there was not an advance care plan in place for the person living with dementia. Even when the prior wishes of the person with dementia were known, the process of decision-making was often fraught with complexity. Discussion Surrogate decision-making on behalf of a person living with dementia is often a difficult process. Advance care planning can play an important role in supporting this process. Healthcare professionals can recognise the challenges that surrogate decision-makers face and support them through advance care planning in a way that suits their needs and circumstances.