This week has been one of the worst in history for the stock market. With the Dow and S&P 500 now down more than 15 percent from recent highs (of only weeks ago!), fears are rising that this pullback might be the precursor to another crisis. So what should we, as investors, do to protect ourselves? Since panic is never the right answer, we need to think through and understand what is really happening—and what is not.