Incidents in production systems are common and downtime is expensive. Applying an appropriate mitigating action quickly, such as changing a specific firewall rule, reverting a change, or diverting traffic to a different availability zone, saves money. Incident localization is time-consuming since a single failure can have many effects, extending far from the site of failure. Knowing how different system events relate to each other is necessary to quickly identify \emph{where} to mitigate. Our approach, Aggregate Comparison of Traces (ACT), localizes incidents by comparing sets of traces (which capture events and their relationships for individual requests) sampled from the most recent steady-state operation and during an incident. In our quantitative experiments, we show that ACT is able to effectively localize more than 99% of incidents.