Title IV of the Clean Air Act Amendments of 1990 established the allowance market system known today as the Acid Rain Program. Initially targeting only sulfur dioxide, Title IV set a decreasing cap on total SO2 emissions for each of the following several years, aiming to reduce overall emissions to 50% of 1980 levels. The program did not begin immediately, but was implemented in two stages: Phase I (starting January 1, 1995) and Phase II (starting January 1, 2000).[6]