This document discusses the importance of measuring the impact of library services in order to demonstrate their value, especially to politicians and managers. It introduces the NHS Impact Toolkit, which provides pragmatic tools like surveys, interviews, case studies and questionnaires to systematically gather information about how library services impact organizational objectives and patient care. The toolkit has been piloted and refined since 2003. Tips are provided on adopting and adapting the toolkit's tools to assess the impact of individual library services.