Stocks have been trading at or near record highs — but not everyone is so certain that the acceleration will continue.

One measure of optimism, fund flows, showed a reversal last month, with fund investors — a group that largely consists of middle-class American retirement savers — pulling more than $2 billion net in May from domestic stock mutual funds and ETFs.

Many professional investors are expressing concern as well. The following five investors, all of whom saw signs of the 2008 financial collapse, are now forecasting declines ahead, many in quite dire terms.

While none should send you screaming for the exits — after all, many experts were making similarly grim predictions a year ago — their jitters should serve as a good reminder to revisit your portfolio and make any needed adjustments to your investing mix, particularly if you’re likely to need the money soon.

When exactly will the good times end?