  Topological defects are involved in a plethora of physical and astrophysical
phenomena. In these lectures, I will review the r\^ ole they could play in the
large-scale structure formation and the anisotropies of the cosmic microwave
background, as well as in various high energy phenomena, including baryon
number asymmetry, ultra-high energy cosmic rays, and gamma ray bursts. I will
then summarize the gravitational effects of cosmic strings. Finally, I will
briefly discuss the r\^ ole of topological defects in brane world cosmology.
