For convenience we give some elements of relativistic quantum mechanics in Section 1.1. We give a number of examples of Lorentz invariant systems in this volume of QI,B. We follow the above steps in each case. This procedure differs from the historical one for the Dirac particle discussed in Chapter 4 but it yields all the usual results. We consider a single spinless particle in Chapter 2, a particle with spin in Chapter 3, a Dirac particle in Chapter 4, a system of particle with spin in Chapter 5 and a simple system involving particle creation and annihilation in Chapter 6. Lists of selected reference books, journal articles and theses follow Chapter 6.