  The topic of cosmic strings provides a bridge between the physics of the very
small and the very large. They are predicted by some unified theories of
particle interactions. If they exist, they may help to explain some of the
largest-scale structures seen in the Universe today. They are `topological
defects' that may have been formed at phase transitions in the very early
history of the Universe, analogous to those found in some condensed-matter
systems --- vortex lines in liquid helium, flux tubes in type-II
superconductors, or disclination lines in liquid crystals. In this review, we
describe what they are, why they have been hypothesized and what their
cosmological implications would be. The relevant background from the standard
models of particle physics and cosmology is described in section 1. In section
2, we review the idea of symmetry breaking in field theories, and show how the
defects formed are constrained by the topology of the manifold of degenerate
vacuum states. We also discuss the different types of cosmic strings that can
appear in different field theories. Section 3 is devoted to the dynamics of
cosmic strings, and section 4 to their interaction with other fields. The
formation and evolution of cosmic strings in the early Universe is the subject
of section 5, while section 6 deals with their observational implications.
Finally, the present status of the theory is reviewed in section 7.
