Assessment is central to identifying needs, making decisions and providing services. Assessment tools have a role in relation to co-ordinating care, communication between professionals and gathering data for monitoring and service improvement. As the health and social care of older people becomes more complex, there is an increased requirement for co-ordinated, effective and efficient assessment. This paper outlines the development of the Northern Ireland Single Assessment Tool (NISAT) for the health and social care of older people. The development involved stakeholders from a wide range of professions, older people and carers. The process included a survey of existing care management assessment tools, various working groups and testing reliability using vignettes and trained actors. Older people were engaged in a music, dance and visual arts project on the theme of assessment to inform the tool development. The components of the tool and their development are reviewed, including considering the role of social work in contributing to specialist assessment as distinct from the role of co-ordinating a holistic assessment process. There are challenges facing co-ordinated assessment processes in health and social care of older people because of the wide variety of care pathways in primary, acute, ‘intermediate’ and community care settings.