Widespread adoption of just-in-time (JIT) production has reduced inventory holdings. This paper measures the frequency of JIT adoption among public firms and quantifies a trade-off created by JIT between firm profitability and vulnerability to supply disruptions. Empirically, JIT adopters experience higher sales and less volatility on average while also exhibiting heightened sensitivity to aggregate supply conditions and weather events faced by their suppliers. I explain these facts in a structurally estimated general equilibrium model of JIT production. Relative to a counterfactual economy without JIT, the baseline model implies higher firm profitability in normal times but a deeper contraction amid a supply disruption. A transition to an equilibrium with less JIT and larger inventory stocks leads to a 4% output loss.