The purpose of this mixed methods study was to describe the cognitive processes/knowledge sources used by Emergency Department (ED) nurses in decision-making activities regarding triage and pediatric pain assessment and management. Deficiencies persist in ED pediatric pain assessment, and management methods or approaches that might help resolve these deficiencies have not been identified previously. Methodology triangulation with sequential use of qualitative- quantitative methods provided a rich description of knowledge sources and cognitive processes used by ED nurses relative to pediatric pain assessment decisions. Based on qualitative results, a set of vignettes was developed to assess ED nurses. Data analysis using ordinal logistic regression with a cumulative logit model identified patient and nurse variables which influence triage acuity decisions. Five common themes emerged from the qualitative data; 1) Age of the child is important, 2) Behavior can tell a lot, 3) Really looking at the patient, 4) Things that help make decisions, and 5) Things that hinder decisions. Ordinal logistic regression analysis of the quantitative data identified predictor variables of infants compared to school-age children, Hispanic ethnicity, moderate number of years of ED experience (11 -20 years) and years of education that were associated with higher triage levels .The implications of this new knowledge include changes in ED triage nurse practice towards pain assessment, and increased awareness of the need for education in use of pain assessment tools. Additional implications include education related to pain management practices by ED physicians and pain medication protocols at triage. This information may enhance triage and care of the pediatric patient experiencing pain, expand the knowledge base of emergency nursing, identify areas in which to implement changes, assist in improving care provided to children experiencing pain, and provide direction for future education, training, and research.