This paper reports some results of a study of practitioners’ perspectives on performance assessment in the field of family support services. Existing empirical work on performance assessment emphasises the perspectives offunders and/or service users. However, practitioners are a key stakeholder in both service delivery and assessment, and consideration of how this group approaches and appropriates performance assessment can maximize its effectiveness, and ensure the incorporation of their practice-based knowledge about service delivery and outcomes. We find that family support workers are committed to understanding the effectiveness of their work, and use a variety of means to attempt to evaluate their own effectiveness. However, these means are rarely systematic, andare unlikely to provide data useful for measures of service economy and efficiency. This may be because their practice consists of processes to which conventional evaluation techniques are ill-suited. The challenge for providers of social services is to find ways to assess the caring work at the heart of their practice in ways which are ‘legible’ to all stakeholders, including government flinders.