Objective: The authors investigated the effect of boot weight and sole flexibility on spatiotemporal gait characteristics and physiological responses of firefighters in negotiating obstacles. Background: Falls and overexertion are the leading causes of fire ground injuries and fatalities among firefighters. There have been few in-depth studies conducted to evaluate the risk factors of falls and overexertion associated with firefighter boots. Method: For the study, 13 female and 14 male firefighters, while wearing full turnout clothing and randomly assigned boots, walked for 5 min while stepping over obstacles. The independent variables included boot weight, sole flexibility, gender, and task duration. Spatiotemporal measures of foot trajectories and toe clearance were determined. Minute ventilation, oxygen consumption, carbon dioxide production, and heart rate were measured. Results: Increased boot weight was found to significantly reduce trailing toe clearance when crossing the 30-cm obstacle. Significant increases in lateral displacement of the foot were found near the end of the 5-min walk compared with the beginning of the task. Increased boot weight significantly increased oxygen consumption. There were significant decreases in oxygen consumption for more flexible soles. Conclusion: Firefighters were more likely to trip over obstacles when wearing heavier boots and after walking for a period of time. Boot weight affected metabolic variables (5% to 11% increases per 1-kg increase in boot weight), which were mitigated by sole flexibility (5% to 7% decrease for more flexible soles). Application: This study provides useful information for firefighters and boot manufacturers in boot selection and design for reducing falls and overexertion.