Standardised health status questionnaires are widely used to obtain subjective assessments of health. However, little research has investigated the meaning of the data they produce. Statistical tests will highlight some problems with the structure and wording of a questionnaire but they cannot shed any light on the way in which respondents interpret questions or their intended meaning when they select a response. Various qualitative techniques are being used within disciplines such as sociology and psychology to test both the language of survey instruments and the cognitive bases of surveys. This paper outlines some of these methods and reports findings from a qualitative research study in the UK with a widely used questionnaire--the Short-Form 36 Health Status Questionnaire. The value of including in-depth, qualitative validation techniques in the development and testing of surveys used to collect subjective assessments of health is clearly demonstrated by the findings of the study.