Triage, the first step in the assessment of emergency department patients, occurs in a highly dynamic environment that functions under constraints of time, physical space, and patient needs that may exceed available resources. Through triage, patients are placed into one of a limited number of categories using a subset of diagnostic information. To facilitate this task and standardize the triage decision process, triage guidelines have been implemented. However, these protocols are interpreted differently by highly experienced (expert) nurses and less experienced (novice) nurses. This study investigates the process of triage; the factors that influence triage decision-making, and how the guidelines are used in the process. Using observations and semi-structured interviews of triage nurses, data was collected in the pediatric emergency department of a large Canadian teaching hospital. Results show that in emergency situations (1) triage decisions were often non-analytic and based on intuition, particularly with increasing expertise, and (2) guidelines were used differently by nurses during the triage process. These results suggest that explicit guideline information becomes internalized and implicitly used in emergency triage practice as nurses gain experience. Implications of these results for nursing education and training, and guideline development for emergency care are discussed.