The objective of this study was to investigate the methodological theoretical framework of the Lean approach in overcrowded emergency services with the implementation of Fast Track. With the integrative review method, it was defined the guiding question: Which Lean principle is related to the application of Fast Track in overcrowded emergency services? The following database were used: Web of Science, PubMed, and Scopus in which 268 records were collected and 14 articles were selected. It was verified that Fast Track is a fast flow that favors the application of the principle of Continuous Flow, essential in emergency services with overcrowding, as it aims to reduce the average length of stay of patients with lower acuity. It was identified in the methodological theoretical framework the demand for more in-depth studies; and that Fast Track is an approach to reducing overcrowding in emergency services associated with the Continuous Flow principle. It was concluded that Fast Track proved to be an effective management strategy in reducing the average length of stay and waiting time of patients, which resulted in improvements in the flow of care and increased patient satisfaction, consequently providing improved performance in the emergency services.