  In 1970 Zel'dovich published a far-reaching paper presenting a simple
equation describing the nonlinear growth of primordial density inhomogeneities.
The equation was remarkably successful in explaining the large scale structure
in the Universe that we observe: a Universe in which the structure appears to
be delineated by filaments and clusters of galaxies surrounding huge void
regions. In order to concretise this impression it is necessary to define these
structural elements through formal techniques with which we can compare the
Zel'dovich model and N-body simulations with the observational data.
  We present an overview of recent efforts to identify voids, filaments and
clusters in both the observed galaxy distribution and in numerical simulations
of structure formation. We focus, in particular, on methods that involve no
fine-tuning of parameters and that handle scale dependence automatically. It is
important that these techniques should result in finding structures that relate
directly to the dynamical mechanism of structure formation.
