A study is made of 4-dimensional Lorentz manifolds which are projectively
related, that is, whose Levi-Civita connections give rise to the same
(unparameterised) geodesics. A brief review of some relevant recent work is
provided and a list of new results connecting projective relatedness and the
holonomy type of the Lorentz manifold in question is given. This necessitates a
review of the possible holonomy groups for such manifolds which, in turn,
requires a certain convenient classification of the associated curvature
tensors. These reviews are provided.Comment: Comments: 23 pages, LaTeX; typos corrected, page 9 last line
corrected to $g'=e^{2\chi}a^{-1}
