  The theory of substitution sequences and their higher-dimensional analogues
is intimately connected with symbolic dynamics. By systematically studying the
factors (in the sense of dynamical systems theory) of a substitution dynamical
system, one can reach a better understanding of spectral and topological
properties. We illustrate this point of view by means of some characteristic
examples, including a rather universal substitution in one dimension as well as
the squiral and the table tilings of the plane.
