Context. Family carers need to be supported in their central role of caring for patients at the end of life, but brief practical tools to assess their support needs have been missing. To address this gap, we developed a brief evidence-based Carer Support Needs Assessment Tool (CSNAT) suitable for everyday practice. Objectives. To assess face, content, and criterion validity of the CSNAT and measure sensitivity to change over time. Methods. Participants were 225 adult carers of patients from six U. K. Hospice Home Care services. Carers were surveyed at baseline and at four-week follow-up using self-completed questionnaires, including CSNAT, standard measures (distress, strain, positive appraisals, preparedness, and global health), help provided with activities of daily living, and patients' symptom levels. Qualitative feedback on CSNAT was sought through 10 pilot carer interviews and professional and carer advisory group input. Results. The CSNAT has good face, content, and criterion validity. CSNAT domains comprehensively covered carer support needs. CSNAT scores showed clear and consistent positive correlations with strain and distress and negative correlations with preparedness for caregiving and global health. There also were clear correlations with help with activities of daily living and some relationships with positive appraisals and symptom burden. The CSNAT's sensitivity to change in relevant domains was similar to other measures. Conclusion. The CSNAT is a valid tool for the direct measurement of carers' support needs. It combines comprehensiveness of content with feasibility of administration and has utility both as a research tool and a tool for everyday palliative care practice.