The chemical and pharmaceutical industries employ many exothermic reactions, and loss of control can lead to thermal runaway. Although control systems are becoming more and more sophisticated, vent systems are still commonly employed. In future years, the increase in environmental issues is likely to outlaw direct atmospheric venting, and there remains the need for passive mitigation of runaway reactions and vented materials. Various options are discussed, including inhibition, quenching of the reactants and separation of the liquid and gas phase before further treatment or venting. Advantages and disadvantages are given. Various currently available design methods are given for separation vessels and quench systems.