We survey literature on ambiguity with an emphasis on recent applications in macroeconomics and finance. Like risk, ambiguity leads to cautious behavior and uncertainty premia in asset markets. Unlike risk, ambiguity can generate first order welfare losses. As a result, precautionary behavior...

Concave hiring rules imply that firms respond more to bad shocks than to good shocks. They provide a unified explanation for several seemingly unrelated facts about employment growth in macro and micro data. In particular, they generate countercyclical movement in both aggregate conditional...