Robert Shiller has successfully called "bubble" twice before, in 2006 on the housing market, and in 1996 on the stock market. Now he has called "bubble" on the stock market, based largely on his famous P/E ratio [Price/Earnings] graph, reproduced above (since June, it has come down a little).
“It looks to me a bit like a bubble again with essentially a tripling of stock prices since 2009 in just six years and at the same time people losing confidence in the valuation of the market,” he said.
Of course, not even Shiller can predict when the alleged bubble will pop. He was a year early on the 2007 housing bubble bursting, and 3 years early on the 2000 stock market bubble. Unlike a lot of economists, Shiller is willing to admit he has no idea when it will pop, if indeed it it a bubble:
... [Shiller] made clear that it remained impossible to time any fall in the market, and cast doubt on whether stocks would drop should the Federal Reserve raise rates later this week.