The S&P 500's Shiller P/E Ratio has surpassed 30 during a continuous bull market for only the sixth time since January 1871, a level that preceded major market declines in all five previous instances. The cyclically adjusted price-to-earnings ratio, which smooths earnings over ten years, reached a peak of 42.84 in early June, the second-highest reading in 155 years. Prior episodes include the 1929 crash that wiped out 89% of the Dow, the dot-com bust that halved the S&P 500, and the 2022 bear market that cut the Nasdaq by a third. While the metric has a flawless record of foreshadowing eventual downturns, historical data also shows that the average S&P 500 bull market lasts more than three times longer than the typical bear market, rewarding long-term investors.