This review article addresses four key questions regarding 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 during recessions, and that exogenous shocks like wars, financial panics, and oil price jumps directly increase uncertainty. Uncertainty also rises endogenously during recessions due to increased volatility. Evidence suggests that uncertainty negatively impacts short-run investment and hiring but may stimulate longer-run innovation. The 2008 jump in uncertainty likely contributed about one-third to the drop in GDP during the Great Recession. The article concludes by discussing the global nature of uncertainty and its implications for economic growth and policy.This review article addresses four key questions regarding 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 during recessions, and that exogenous shocks like wars, financial panics, and oil price jumps directly increase uncertainty. Uncertainty also rises endogenously during recessions due to increased volatility. Evidence suggests that uncertainty negatively impacts short-run investment and hiring but may stimulate longer-run innovation. The 2008 jump in uncertainty likely contributed about one-third to the drop in GDP during the Great Recession. The article concludes by discussing the global nature of uncertainty and its implications for economic growth and policy.