Topology of cosmic domains and strings [1], written in 1976, launched the field of cosmic topological defects, and was seminal for the enormous expansion in research linking particle physics to cosmology in the last few decades. Topological defects are extended structures in field theories, which exist in continuous ordered media, and are also present in models of particle physics which extend the Standard Model. In our three dimensions of space, these structures may have their energy concentrated around points, lines, or planes. For example, superfluid Helium can support linear structures called vortices, around which the fluid circulates without viscosity. A prototype for the Standard Model of particle physics known as the Georgi-Glashow model exhibits spherically symmetric topological defects from which magnetic field lines emerge -these defects are therefore magnetic monopoles.The paper showed how to predict what topological defects would form at a symmetry-breaking phase transition in the early universe, and also made an estimate of the average separation of the defects. All one needs to know are the symmetries of the ground state before and after the phase transition: the mathematical theory of topology can then be applied to calculate what structures are allowed. More specifically, n-dimensional objects will form in d space dimensions if a certain topological quantity, the homotopy group of order d − n − 1, is non-trivial -that is, is larger than the trivial group consisting of the identity element only.The paper led to some immensely important developments. It was one of the seeds of the theory of cosmic inflation, as the lack of any evidence for magnetic monopoles in the universe meant that there was a contradiction between the idea of Grand Unification of forces (taken very seriously in the late 1970s) and the standard hot Big Bang. Inflation -a period of accelerated expansion -was partly motivated by its feature of diluting any population of monopoles to unobservably low densities [2].Kibble's proposal that linear topological defects -cosmic strings -could produce density perturbations was taken up by Zel'dovich [3] and Vilenkin [4], who used strings to construct a theory of galaxy formation. For a while, strings rivalled the currently accepted one based on the quantum fluctuations during inflation. It is interesting that Kibble himself was pessimistic about cosmic strings having any observable consequence,