I think a useful gauge of market valuation is the median price to revenue ratio for the S&P 500 companies.  In 1987 it was around 1.0.  In 1999 it was close to 1.7.  In 2007 it was almost 1.9.  Today it is very close to 2.5.  Using this view of valuation, markets are overvalued by about 30% vs. 2007. We all know that happened then.  Will it happen again?  For sure.  When, and what the catalyst will be, is anybody's guess.  I've seen this song covered many, many times.  I am being very cautious now, and may end up leaving a lot of money on the table as a result.  But one thing I've learned over the years is that while it's great to make a lot of money,  it's much better not to lose a lot.