PURPOSE: The purpose of this research was to describe how nurses in intensive care units (ICU) work. METHOD: A total of 18 ICU nurses participated in the research. The data was collected through individual in-depth interviews and analyzed by grounded theory method using NUDIST 4.0 software program. RESULTS: Three different patterns regarding nursing performance among ICU nurses were identified. These are 1) nursing performance of nurses who perform excellently, 2) nursing performance of nurses who do not perform well because of their lack of experience, and 3) nursing performance of nurses who do not perform well in spite of their good years of experience. These three different nursing performances were described in terms of seven different categories; 1) assessing and monitoring nursing problems, 2) clinical decision making, 3) interpersonal relationships, 4) holistic care, 5) technical skills, 6) problem solving, 7) working independently and creatively. This study also identified two intervening factors that influenced the advancement of their expertise. CONCLUSION: The results of this study might help nurse managers in developing continuing educational programs for inexperienced nurses or those nurses not performing well to become experts by a deeper understanding of the nature of nursing performance and the factors that influence nursing performance in ICU settings.