  Recent research has severely constrained the standard ``defect'' models of
cosmic structure formation. Here I discuss the nature of the problems with
defect models, and place this discussion in the context of the big picture of
cosmic structure formation. In particular, I classify models of cosmic
structure formation as either ``causal'' or ``acausal'', and ask whether the
problems with the defect models extend to all other causal models. I argue that
determining the causal nature of the primordial perturbations is within the
reach of modern cosmology, and that such a determination would yield deep
insights into the very early Universe.
