
Source: en.wikipedia.org
The red line charts the S&P500's price to earning ratio for the past 130 years. When P/E ratios are high the market is overpriced. When they are low the market is underpriced. Even after the recent crash, P/E ratios still look higher than historical averages. The black line shows interest rates which freaked out in the 1970s but are back to normal so you can ignore the black line.