An extended bull market can tempt even the savviest investors into abandoning their long-term discipline. Resisting the impulse to switch horses in the middle of the race is hard, but necessary—the most important trait of successful investors is their ability to maintain discipline in sticking to a long-term strategy during good times and bad. Diversified portfolios—structured to earn returns comparable to their rate of spending at tolerable levels of risk—have benefitted long-term investors and grown their purchasing power for decades, and we have no reason to expect a different outcome when today’s bull market inevitably corrects.