So far, we have focused on the description of particle systems evolving in a non-relativistic context. This means that the typical speeds v involved are significantly lower than the speed of light in vacuum ( \(v \ll c\) ), and that the typical momenta p and energies E are, respectively, less than the order of magnitude mc of the relativistic momenta and the order of magnitude \(mc^2\) of the relativistic energies (for a system of non-zero mass m). In this chapter, we will study how to formulate quantum mechanics in a more general setting, valid in the relativistic context. This theoretical framework, named relativistic quantum mechanics,relativistic quantum mechanics is one of the basic tools of particle physics. Relativistic quantum mechanics was developed following the observation that quantum mechanics, as presented in the previous chapters, does not satisfy the principles of special relativitySpecial relativity. It is thus necessary to generalise the concepts of quantum physics in order to be able to predict all experimental observations, both in the non-relativistic and relativistic cases.