The share of corporate profits in the U.S. collected by state governments via the corporate income tax has fallen sharply in the past quarter century. Some commentators have even referred to this as the "disappearance" of the state corporate income tax (SCIT). Such claims, of course, are an exaggeration--after all, a longer perspective reveals that the share of profits collected by state corporate income taxes was actually lower in the 1960s than it is now. Nonetheless, state public finance experts and state policymakers surely are correct in noting that, since around 1980, corporate income taxes have become an increasingly smaller share of total state tax revenues and a smaller share of businesses' costs. ; This Economic Letter attempts to unravel the mystery of falling state corporate income taxes by analyzing the primary determinants of these taxes and reviewing how they have changed in the last 25 years.