BACKGROUND Around the world, registered nurses are working increasing amounts of overtime. This is particularly true in critical care environments, which experience unpredictable fluctuations in patient volume and acuity, combined with a need for more specialized nurses. OBJECTIVE To explore critical care nurses’ reasons for working or not working overtime. METHODS A semistructured interview guide was used to interview 28 frontline nurses from 11 critical care units in Ontario, Canada. Analysis was guided by Thorne’s interpretive description methodology. RESULTS Participants’ reasons for working overtime included (1) financial gain (96% of participants); (2) helping and being with colleagues (68%); (3) continuity for nurses and patients (39%); and (4) accelerated career development (39%). Their reasons for not working overtime were (1) feeling tired and tired of being at work (50%); (2) having established plans (71%); and (3) not receiving enough notice (61%). CONCLUSIONS Findings from this study provide important variations and extension of existing literature on the topic, and appear to be the first reported in Canadian critical care units. Additional research is required to understand administrative decision-making processes that lead to the use of overtime.