The Office of Research and Development of the U.S. EPA supports a methods development program for FTIR-based, open path monitoring of trace gases in the air. This type of system provides an estimate of the average concentration of individual trace gases over distances up to approximately one kilometer by interpreting changes in the infrared absorption spectrum. The EPA program has four objectives. The first is to develop a guidance document to explain the nature of these systems, their uses, and their performance-limiting features. The second is to provide a method decribing recommended procedures for operating the systems to take data; the third is to support an international effort by L'Organisation Internationale de Metrologie Legale (OIML) to specify acceptale performance levels for commercial FTIR-based systems, and the last is to support the development of a library of reference spectra in the infrared at the National Institute of Standards and Technology (NIST).