  Cracks are the major vehicle for material failure, and often exhibit rather
complex dynamics. The laws that govern their motion have remained an object of
constant study for nearly a century. The simplest kind of dynamic crack is a
single crack that moves along a straight line. We first briefly review current
understanding of this "simple" object. We then critically examine the
assumptions of the classic, scale-free, theory of dynamic fracture, and note
when it works and how it may fail if certain of these assumptions are relaxed.
A number of examples is provided, where the introduction of physical scales
into this scale-free theory profoundly affects both a crack's structure and the
resulting dynamics.
