  We review the non-trivial issue of the relativistic description of a quantum
mechanical system that, contrary to a common belief, kept theoreticians busy
from the end of 1920s to (at least) mid 1940s. Starting by the well-known works
by Klein-Gordon and Dirac, we then give an account of the main results achieved
by a variety of different authors, ranging from de Broglie to Proca, Majorana,
Fierz-Pauli, Kemmer, Rarita-Schwinger and many others.
  A particular interest comes out for the general problem of the description of
particles with \textit{arbitrary} spin, introduced (and solved) by Majorana as
early as 1932, and later reconsidered, within a different approach, by Dirac in
1936 and by Fierz-Pauli in 1939. The final settlement of the problem in 1945 by
Bhabha, who came back to the general ideas introduced by Majorana in 1932, is
discussed as well, and, by making recourse also to unpublished documents by
Majorana, we are able to reconstruct the line of reasoning behind the Majorana
and the Bhabha equations, as well as its evolution. Intriguingly enough, such
an evolution was \textit{identical} in the two authors, the difference being
just the period of time required for that: probably few weeks in one case
(Majorana), while more than ten years in the other one (Bhabha), with the
contribution of several intermediate authors.
  Majorana's paper of 1932, in fact, contrary to the more complicated
Dirac-Fierz-Pauli formalism, resulted to be very difficult to fully understand
(probably for its pregnant meaning and latent physical and mathematical
content): as is clear from his letters, even Pauli (who suggested its reading
to Bhabha) took about one year in 1940-1 to understand it. This just testifies
for the difficulty of the problem, and for the depth of Majorana's reasoning
and results.
