How expensive is the US stock market? Have the strong gains since the financial crisis of 2008 built yet another massive bubble that will require a correction? Some investors fear that recent stock market record levels are a sign that this is the biggest bubble ever. Of course, such a simplistic comparison of stock prices is flawed both because stock prices are nominal and therefore likely to go up over time and also because they are driven by (real) earnings which are also likely to increase as (real) GDP increases. To correct for these trends we can do a simple adjustment and look instead at the price-to-earnings ratio. I will use here the one constructed by Robert Shiller (although some ague that recently the index could be less informative because of the way earnings are being calculated ). The P/E ratio shows that the end of the 90s bubble was by far the period were stocks looked the most expensive relative to earnings. What do P/E ratios look like today? On the expensive side. W...