BACKGROUND: Contemporary nursing literature emphasises the desirability of clinical nurses being "knowledgeable". However, the need for nurses constantly to acquire more knowledge is reiterated. Lack of knowledge is seen to underlie an array of professional problems. Little is known of how nurses themselves understand what it means to practise knowledgeably. OBJECTIVE: To explore critical care nurses' understandings of knowledgeable practice and its relationship to being a "good nurse". METHODOLOGY: A poststructuralist framework informed the study. The study participants were 12 critical care nurses. Data were generated through three individual focused interviews with each participant. Data analysis involved deconstruction of the interview texts to reveal participants' discourses of knowledgeable practice and the implications of these discourses for their subjectivity and for their work. FINDINGS: A discourse of knowledgeable practice was revealed as central to participants' sense of identity as "good nurses". Participants believed their knowledge resided in their heads ("knowing why") and in their hands ("knowing how"). Fluency of action, which was achieved and maintained by frequent repetition of activities, contributed to their sense of being knowledgeable. Participants described being excluded from knowledge in some instances. In general, however, "actual" knowledge was of less importance than was being positioned, by themselves and others, as knowledgeable. This positioning was frequently undermined by other staff, both medical and nursing. Analysis revealed that the discourse of knowledgeable practice was underpinned by a dichotomy of ignorant/knowledgeable, in which "ignorant" was the dominant category; hence, nurses were assumed to be ignorant until they could "prove" otherwise. CONCLUSIONS: The findings contest the notion, espoused in nursing literature, that acquisition of knowledge can "empower" nurses, thus providing the solution to problems they may experience. Rather, strategies are required that challenge and disrupt relations of power that construct nurses as "ignorant".