Short-time work programs were revived by the Great Recession. To understand their operating mechanisms, we …rst provide a model showing that short-time work may save jobs in …rms hit by strong negative revenue shocks, but not in less severely-hit …rms, where hours worked are reduced, without saving jobs. The cost of saving jobs is low because short-time work targets those at risk of being destroyed. Using extremely detailed data on the administration of the program covering the universe of French establishments, we devise a causal identi…cation strategy based on the geography of the program that demonstrates that short-time work saved jobs in …rms faced with large drops in their revenues during the Great Recession, in particular when highly levered, but only in these …rms. The measured cost per saved job is shown to be very low relative to that of other employment policies.