In this retrospective cohort study, critical events occurred in approximately 1 in 15 urgent interfacility transports. Patients are particularly vulnerable to adverse events during transfer due to limited resources and the physical stresses of being transported. Previous studies have examined critical events during air medical transport, but few have explored urgent land-based interfacility transfers. New hypotension was relatively common during transit in this study of more than 5000 adult critical care transports. Patients with mechanical ventilation or baseline hemodynamic instability were more likely to experience in-transit adverse events. An AHRQ WebM&M commentary reviews the dangers associated with transferring critically ill patients even within the same hospital.