Topological defects formed at symmetry-breaking phase transitions play an important role in
many different fields of physics. They appear in many condensed-matter systems at low
temperature examples include vortices in superfluid helium-4 a rich variety of defects in
helium-3 quantized mag netic flux tubes in type-II superconductors and disclination lines
and other defects in liquid crystals. In cosmology unified gauge theories of particle
interactions suggest a sequence of phase transitions in the very early uni verse some of which
may lead to defect formation. In astrophysics defects play an important role in the dynamics
of neutron stars. In 1997 the European Science Foundation started the scientific network
Topological defects headed by Tom Kibble. This network has provided us with a unique
opportunity of establishing a collaboration between the representatives of these very different
branches of modern physics. The NATO-ASI (Advanced Study Institute) held in Les Houches in
February 1999 thanks to the support of the Scientific Division of NATO the European Science
Foundation and the CNRS represents a key event of this ESF network. It brought together
participants from widely different fields with diverse expertise and vocabulary fostering the
exchange of ideas. The lectures given by particle physicists cosmologists and condensed matter
physicists are the result of the fruitful collaborations established since 1997 between groups
in several European countries and in the U.S.A.
