Objectives: Capture change in family members' experiences as they look after patients during chemotherapy, and understand variability in their needs for support. Methods: Longitudinal digitally-recorded qualitative semi-structured interviews with family carers at the beginning, mid-point, and end of treatment. Twenty-five family members (17 women, 8 men), mean age 53, were interviewed. Fifteen participants were supporting a relative having chemotherapy with curative intent, and 10 a patient receiving palliative chemotherapy. They were recruited from two UK locations: a regional cancer centre in Southampton and a comprehensive cancer centre in London. Sixty-three interviews were conducted in total, and the data were analysed using Framework Analysis. Findings: Three themes were generated from the data: Changing lives, Changing roles ; Confidence in caring , and Managing uncertainty. These captured family carers' evolving needs and sense of confidence in caregiving during chemotherapy. Carers reported considerable anxiety at the outset of treatment which persisted throughout. Anxiety was underpinned by fears of disease recurrence or progression and concerns about treatment outcomes. Conclusions: This study presents original fine-grained work that captures the changes over time in family carers' experiences of chemotherapy and their adaptation to caregiving. It provides fundamental evidence of the challenges that cancer carers face during patients' treatment; evidence that can be used as a basis for carer assessment and to build much-needed carer interventions. Oncology nurses should assess carers': ability to care; needs for information and support to prepare them for this; wellbeing over time; and, any support they may require to prevent them from becoming overburdened.