This review article addresses four key questions about uncertainty: (i) what are the stylized facts about uncertainty over time; (ii) why does uncertainty vary; (iii) do fluctuations in uncertainty matter; and (iv) did higher uncertainty worsen the Great Recession of 2007-2009? The article finds that both macro and micro uncertainty rise sharply in recessions and fall in booms. Uncertainty varies heavily across countries, with developing countries having about a third more macro uncertainty than developed countries. Exogenous shocks like wars, oil price jumps, and financial panics increase uncertainty, and uncertainty also endogenously rises during recessions. Evidence suggests uncertainty is damaging for short-run investment and hiring, but may stimulate longer-run innovation. During the Great Recession, a large jump in uncertainty in 2008 potentially accounted for about one third of the drop in GDP. The article also discusses the mechanisms behind uncertainty variation, including the role of exogenous shocks, endogenous uncertainty, policy uncertainty, and micro-uncertainty. It highlights the negative short-run impact of uncertainty on growth, investment, hiring, and consumption, while noting some potential positive effects on R&D. The article concludes that uncertainty played a significant role in the depth and recovery from the Great Recession, with policy uncertainty remaining high due to ongoing fiscal debates. Overall, uncertainty is strongly countercyclical, rising in recessions and falling in booms, and is substantially higher in developing countries.This review article addresses four key questions about uncertainty: (i) what are the stylized facts about uncertainty over time; (ii) why does uncertainty vary; (iii) do fluctuations in uncertainty matter; and (iv) did higher uncertainty worsen the Great Recession of 2007-2009? The article finds that both macro and micro uncertainty rise sharply in recessions and fall in booms. Uncertainty varies heavily across countries, with developing countries having about a third more macro uncertainty than developed countries. Exogenous shocks like wars, oil price jumps, and financial panics increase uncertainty, and uncertainty also endogenously rises during recessions. Evidence suggests uncertainty is damaging for short-run investment and hiring, but may stimulate longer-run innovation. During the Great Recession, a large jump in uncertainty in 2008 potentially accounted for about one third of the drop in GDP. The article also discusses the mechanisms behind uncertainty variation, including the role of exogenous shocks, endogenous uncertainty, policy uncertainty, and micro-uncertainty. It highlights the negative short-run impact of uncertainty on growth, investment, hiring, and consumption, while noting some potential positive effects on R&D. The article concludes that uncertainty played a significant role in the depth and recovery from the Great Recession, with policy uncertainty remaining high due to ongoing fiscal debates. Overall, uncertainty is strongly countercyclical, rising in recessions and falling in booms, and is substantially higher in developing countries.