  Cosmic strings are linear concentrations of energy that may be formed at
phase transitions in the very early universe. At one time they were thought to
provide a possible origin for the density inhomogeneities from which galaxies
eventually develop, though this idea has been ruled out, primarily by
observations of the cosmic microwave background (CMB). Fundamental strings are
the supposed building blocks of all matter in superstring theory or its modern
version, M-theory. These two concepts were originally very far apart, but
recent developments have brought them closer. The `brane-world' scenario in
particular suggests the existence of macroscopic fundamental strings that could
well play a role very similar to that of cosmic strings.
  In this paper, we outline these new developments, and also analyze recent
observational evidence, and prospects for the future.
