Offer your thoughts or opinion on the input financial query or topic using your financial background.

Does it make sense to buy an index ETF (e.g. S&P 500) when the index is at an all-time high?
Here is, from Yahoo Finance, the S&P 500 over the last ~60 years (logarithmic scale):  The behavior since ~2000 has been weird, by historical standards. And it's very easy, looking at that graph, to say "yes! I would have made so much money had I invested in March '09!". Of course, back in March '09, it wasn't so clear that was the bottom. But, yes, over the last 10 years or so, you could have made more money by adopting a rule that you'll accumulate cash in a FDIC (or similar) insured savings account, and dump it into an S&P index fund/ETF when the index is n% off its high. Of course, if you look at the rest of the chart, that strategy looks a lot less promising. Start in the early 80's, and you'd have held cash until the crash in 2000. Except for the recent weirdness, the general trend in the S&P 500 (and stock markets in general) has been upward. In other words, to a first-order approximation, the S&P 500 is always at an all-time high. That's just the general trend.