This paper discusses the nature of the instruments used to assess students' and teachers' conceptions of the nature of science during the last four decades. Two salient points emerge from critically reviewing the various research emphases and associated assessment instruments specifically related to the nature of science. The first is to question the validity of much of the research presented on the grounds that (a) assessment instruments are interpreted in a biased manner, and (b) some assessment instruments appear to be poorly constructed. Although such comments are well founded, it is important to note that the research conclusions were unusually uniform regardless of the particular instrument used in an investigation. The second point is a more critical concern about the “traditional” paper and pencil approach to the assessment of an individual's understanding of the nature of science. Although not a new insight, discrepancies between the interpretation of a written response to a “traditional” instrument (e.g., multiple choice assessment) and the intent of the respondent have been well documented. It is suggested that the current educational research shift toward more qualitative, open-ended approaches to assessment of individuals' understanding (of any concept) be applied to the assessment of individuals' nature of science conceptions.