In July 2002, U.S.E.P.A. promulgated regulations containing new emissions standards for ethylene processes based on “maximum achievable control technology” or MACT. These regulations and subsequent amendments became effective July 12, 2005. This paper provides a brief overview of the Ethylene MACT regulatory requirements, a discussion of the joint industry-EPA effort to develop a compliance manual, and a look at the regulatory horizon for the ethylene industry.