The S&P 500's Shiller P/E ratio has climbed to 42.84, approaching the all-time high of 44.19 set in December 1999 just before the dot-com bubble burst. This valuation metric, backtested to 1871, has only exceeded 30 during a continuous bull market on five prior occasions, each followed by a major downturn including the Great Depression and the dot-com crash. While the current reading signals historically unsustainable levels, analysts note that over every rolling 20-year period since 1900 the S&P 500 has delivered positive total returns, rewarding long-term investors.