Although the advance planning for it was minimal by today’s standards, a large integrated federal and state environmental monitoring response was made to the Three Mile Island Unit 2 accident. In particular, major resources were committed by the U.S. Department of Energy (DOE). They include the Brookhaven National Laboratory-based Radiological Assistance Program, the Atmospheric Release Advisory Capability, and the Aerial Measurements System, with backup personnel from other DOE national laboratories. Additional resources were provided by the Environmental Protection Agency. The monitoring effort included plume tracking, field environmental monitoring and sampling, sample analysis, and dose assessment. The ranges of the installed plant monitors having been exceeded, these data were important for establishing the nuclides and their quantities in the daily releases from the plant during the first few weeks after the accident. In particular, it was established that the continuing releases consisted almost entirely of radiogases, with very small quantities of radioiodines. The highest measured ground-level dose rate was 1.3 × 102 C/kg (50 mR/h) and the largest concentration of 131I <3.7 × 10−6 Bq/cm3 (<1 × 10−10 µCi/cm3). From DOE population dose assessment, the highest individual dose appears to have been <1 mSv/h (<100 mR/h) and the total population dose 20 person-Sv (∼2000 person-rems). This largely ad hoc response became the basic model for today’s Federal Radiological Monitoring and Assessment Program, which would be put into operation should a major accident occur at a U.S. nuclear facility.