  In these notes we provide a pedagogical introduction to the subject of
tachyon condensation in Witten's cubic bosonic open string field theory. We use
both the low-energy Yang-Mills description and the language of string field
theory to explain the problem of tachyon condensation on unstable D-branes. We
give a self-contained introduction to open string field theory using both
conformal field theory and overlap integrals. Our main subjects are the Sen
conjectures on tachyon condensation in open string field theory and the
evidence that supports these conjectures. We conclude with a discussion of
vacuum string field theory and projectors of the star-algebra of open string
fields. We comment on the possible role of string field theory in the
construction of a nonperturbative formulation of string theory that captures
all possible string backgrounds.
