Planning for retirement is challenging in just about every aspect, from saving enough to investing well and controlling spending. But one challenge stands out as being the hardest to address: the uncertainty of how long you'll live.

In the following video, Dan Caplinger, The Motley Fool's director of investment planning, looks at the question of life expectancy and how to plan for it. Dan notes that essentially, unknown life expectancy forces you to plan for all contingencies, inevitably making it impossible to match up spending and saving perfectly. Dan notes that immediate annuities can help you eliminate some of the risk of outliving your money, with AIG (NYSE:AIG), Prudential (NYSE:PRU), and MetLife (NYSE:MET) among the top sellers of products that guarantee lifetime income. But Dan also discusses the challenges of using those products, which can often be hard to understand and laden with fees. Dan concludes that while there's no easy solution, being aware of the problem of longevity risk can help you take early action if you start to run out of money in retirement.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.