A review of analytical studies for the formation of triple points on shock fronts and for the cellular structures of gaseous detonations is presented. The analyses concern two opposite limiting cases, strong and weak shocks in the Newtonian approximation. In the first case the gas flow is mainly controlled by quasi-isobaric entropy-vorticity waves while the flow is transonic in the second case. The cellular detonations are first investigated near the multidimensional instability threshold. The analysis is extended to strongly unstable detonations in a second step.