Accusations of bias are not uncommon in the social sciences. However, the term ‘bias’ is by no means straightforward in meaning. One problem is that it is ambiguous. Sometimes, it is used to refer to the adoption of a particular perspective from which some things become salient and others merge into the background. More commonly, ‘bias’ refers to systematic error: deviation from a true score, the latter referring to the valid measurement of some phenomenon or to accurate estimation of a population parameter. The term may also be used in a more specific sense, to denote one particular source of systematic error: that deriving from a conscious or unconscious tendency on the part of a researcher to produce data, and/or to interpret them, in a way that inclines towards erroneous conclusions which are in line with his or her commitments. In either form, the use of ‘bias’ to refer to systematic error is problematic. It depends on other concepts, such as ‘truth’ and ‘objectivity’, whose justification and role have been questioned. In particular, it seems to rely on foundationalist epistemological assumptions that have been discredited. And the various radical epistemological positions that some social scientists have adopted as an alternative either deny the validity of this concept of bias, explicitly or implicitly, or transform it entirely. We will argue, however, that while it is true that abandonment of a foundationalist conception of science has important implications for the meaning of ‘bias’ and its associated concepts, they are defensible; indeed, they form an essential framework for research as a social practice. In this context, we shall examine error as a matter of collegial accountability, and define ‘bias’ as one of several potential forms of error. We conclude by pointing to what we see as the growing threat of bias in the present state of social research.