This chapter provides an overview of H 2 ignition and safety-related questions, to be addressed in the development of future H 2 thermal engines. Basics of H 2 ignition phenomena are covered in the first part, including the well-known branchedchain oxidation reactions described by Semenov & Hinshelwood, as well as useful analytical derivations of induction delay times. The second part provides an overview of classical canonical limit problems, including the explosion-limit (,) diagram, the propagation limits of both deflagrations and detonations, and shock-induced or thermal-induced ignitions. The two remaining parts address two opposite but complementary questions: how to ignite a H 2 engine, and how to prevent hazardous H 2 ignition. In the former, a list of available technologies is offered, while in the latter, simplified models are presented to predict ignition hazards from cold-flow numerical simulations.