The economy may be healthy, but the cynics abound.

Even though many say conditions remain strong, several large investors see the risks of a recession rising, with half of chief financial officers expecting one to strike next year, according to a recent survey.

A survey of chief executives at a Yale University summit conference last week found that almost half thought the country could find itself in a recession as soon as the end of the month.

It’s anyone’s guess when the good times will end, but in a complicated, volatile world it’s not a question of if a recession is coming, but when.

Here’s a brief guide to what you should know about recessions and why some people are talking about the next one now.