  Let $M = M_0 \times \R^2$ be a pp--wave type spacetime endowed with the
metric $<\cdot,\cdot>_z = <\cdot,\cdot>_x + 2 du dv + H(x,u) du^2$, where
$(M_0, <\cdot,\cdot>_x) $ is any Riemannian manifold and $H(x,u)$ an arbitrary
function. We show that the behaviour of $H(x,u)$ at spatial infinity determines
the causality of $M$, say: (a) if $-H(x,u)$ behaves subquadratically (i.e,
essentially $-H(x,u) \leq R_1(u) |x|^{2-\epsilon} $ for some $\epsilon >0$ and
large distance $|x|$ to a fixed point) and the spatial part $(M_0,
<\cdot,\cdot>_x) $ is complete, then the spacetime $M$ is globally hyperbolic,
(b) if $-H(x,u)$ grows at most quadratically (i.e, $-H(x,u) \leq R_1(u)
|x|^{2}$ for large $|x|$) then it is strongly causal and (c) $M$ is always
causal, but there are non-distinguishing examples (and thus, non-strongly
causal), even when $-H(x,u) \leq R_1(u) |x|^{2+\epsilon} $, for small $\epsilon
>0$.
  Therefore, the classical model $M_0 = \R^2$, $H(x,u) = \sum_{i,j} h_{ij}(u)
x_i x_j (\not\equiv 0)$, which is known to be strongly causal but not globally
hyperbolic, lies in the critical quadratic situation with complete $M_0$. This
must be taken into account for realistic applications. In fact, we argue that
$-H$ will be subquadratic (and the spacetime globally hyperbolic) if $M$ is
asymptotically flat. The relation of these results with the notion of
astigmatic conjugacy and the existence of conjugate points is also discussed.
