Companies have seen high profit margins in 2017, and those profit margins are likely still rising. High profit margins have generated 10.2 percent growth on the S&P index during the second quarter of 2017. This 10.2 percent S&P growth occurred even as there was revenue growth of just 5.1 percent. The stock rally this year has largely been caused by incremental margins generating positive earnings surprises, even though revenues were forecast accurately by the market. However, looked at in a larger historical context, there is a big risk that margins may revert back to longer-run averages, especially as the trailing fourth-quarter S&P 500 operating margins were at 10.6 and 10.8 during different quarters in 2017.This is compared with under 8.0 percent in the 1990s and 9 percent in 2006, when these margins hit their previous peak. Corporate tax reform could serve as a counterweight if corporate margins, on average, revert back to longer-run averages.