  I review the paper of Majorana about relativistic particles with arbitrary
spin written in 1932. The main motivation for this papers was the
dissatisfaction about the negative energy solutions of the Dirac equation. As
such, the paper became immediately obsolete due to the almost contemporaneous
discovery of the positron. However, for the first time, the unitary
representations of the Lorentz group were introduced. Majorana considered two
particular representations (named, after him, Majorana representations) which
enjoy many interesting properties. A discussion about the reasons for its
revival in the 60's is presented.
