This technical report presents the 2021 ESO EMS Index Mid-Year Update, which analyzes data from over 4.6 million 911 records collected between January 1, 2021, and June 30, 2021. The report compares these metrics to those from the previous year, specifically focusing on six key performance indicators, including stroke assessment, ketamine administration, and overdose rates. The objective of the Index is to provide EMS leaders with comparative data to inform their practices and improve patient outcomes. The report notes a slight increase in the overdose rate from 2.7% in 2020 to 2.8% in the first half of 2021, while other metrics such as stroke assessment completion have shown improvement. The document emphasizes the importance of accurate data collection and documentation in enhancing EMS performance and patient care. It also discusses the role of the ESO Data Collaborative in gathering and analyzing this data to support EMS agencies across the United States.