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Does it make sense to buy an index ETF (e.g. S&P 500) when the index is at an all-time high?
In other words, to a first-order approximation, the S&P 500 is always at an all-time high. I'm going to run with this observation a bit. The crash of '87 was remarkable. It was a drop of 1/3 in a short time, yet, when one looked at the year, the Dow was up nearly 5% with dividends included. A one-year Rip Van Winkler would have woken up thinking it an unremarkable year.  I actually recall a conversation I had on Aug 25th 1987. I was discussing the market with a colleague over lunch, and while I didn't call the top that day, I remarked that it didn't matter much, that 5-10 years later just staying in the market would have been the right thing.    Compare this 87-95 chart to the longer term chart derobert shows. In his chart, this is all but a blip. In my chart you can see it took about 3-1/2 years to be in the black, as the market then shot up from there. A dollar cost averager would not have bought at that short term high, well not more than a tiny bit.  The best I can do to conclude is to say I'd never just buy in all at once. You buy in over time, X% of your income each month, and if you have a chunk to invest, smooth it out over a few years.