This chapter deals with two main aspects of reaction engineering involved in process safety: reactive hazard and reactor schemes. The latter is traditionally included in chemical engineering courses, however many hazard scenarios require to be modelled as ideal reactors to be properly approached and resolved. For this reason ideal reactors have been dealt with, and specific examples have been included, with the aim to demonstrate the effectiveness of the approach. Reactive hazard implications have also been treated, focusing on the various effects of reactive scenarios potentially resulting in serious outcomes. Energy escalation related to combustion, solution, and neutralisation has been presented, along with pyrophoric scenarios. Practical data, and calculation examples about reactivity and chemical reactions of substances more frequently met in process safety engineering, have also been included in this chapter. As for the previous fundamental concepts, these data are not aimed to fully cover the complex and broad world of chemical hazards. The intent is to provide the process safety engineer with some useful concepts and calculation procedures within the frame of the most usual chemical scenarios. For a full coverage of the subjects discussed here, the following literature reference source can be adopted, among many other very good textbooks.Chemical hazards and reactivity data:-Bretherick and Urben (2006)-Johnson et al. (CCPS, 2003)Reactor engineering:-Levenspiel (1999)-Froment et al. (2011)-Satterfield (1970)