Most explosive mixtures can display flame front velocities several orders of magnitude greater. These flames exceed the characteristic sound speed of the gas and are termed detonations. If longitudinal instability occurs then galloping detonations are observed in which the detonation wave repeatedly decays and then jumps back to its original velocity. The occurrence of exothermic reaction in the reaction zone renders the one-dimensional system unstable. All stable detonations appear to show complex, three-dimensional structure. For the majority of exothermic reaction mixtures, two quite stable combustion waves are possible, a propagating flame and a detonation. Since detonations can cause severe damage, it is important to understand how detonation may be initiated. In order to understand the chemical processes in flames in a quantitative way, it is essential to obtain kinetic data on elementary reactions over very wide ranges of temperatures. Initiation by electric spark can lead either to a normal premixed flame or to a detonation depending on conditions.