  The theme of this book chapter is to discuss algorithms for identifying and
reconstructing groups and clusters of galaxies out of the general galaxy
distribution. I review the progress of detection techniques through time, from
the very first visual-like algorithms to the most performant geometrical
methods available today. This will allow readers to understand the development
of the field as well as the various issues and pitfalls we are confronted with.
This essay is drawn from a talk given by the author at the conference "The
World a Jigsaw: Tessellations in the Sciences" held at the Lorentz Center in
Leiden. It is intended for a broad audience of scientists (and so does not
include full academic referencing), but it may be of interest to specialists.
