Lithium-ion batteries are used in a wide range of applications, from small consumer products and electric vehicles to large stationary energy storage systems and electrically propelled ships. The increased use of lithium-ion batteries for energy storage systems has put an emphasis on battery safety. Upon battery failure, e.g. due to external heating or an internal short circuit, material decomposition and accelerated exothermic reactions may result in a thermal runaway. Thermal runaway in lithium-ion batteries generally means the production of large amounts of flammable gas which poses an explosion risk. To mitigate explosions and to enable safety evaluation and design of appropriate and rightfully dimensioned safety features, such as ventilation, the gas release characteristics are of great importance. In this paper, gas characteristics from thermal runaway in lithium-ion battery cells are evaluated. The gas characteristics, such as the gas production rate, gas volumes and chemical composition are evaluated for more than 80 battery cell tests. The chemical composition was analyzed using multiple techniques to assess the consistency of the obtained data. The main components formed during thermal runaway are carbon dioxide, carbon monoxide, hydrogen and various hydrocarbons. The total volume of gas produced, normalized to the rated electrical energy of the cell, varies typically between 0.1 and 0.7 L/Wh. Results show that the cell type, cell size, state-of-charge and even the thermal runaway trigger method influence the gas characteristics. Furthermore, explosion mitigation strategies for large battery systems focusing on ventilation and ventilation strategies are presented. Finally, safety aspects related to the battery cell and system design, such as choice of cell chemistry, thermal barriers, and routes for safe evacuation of thermal runaway vent gas are discussed.