  Given a D-brane background in string theory (or equivalently boundary
conditions in a two-dimensional conformal field theory), classical solutions of
open string field theory equations of motion are conjectured to describe new
D-brane backgrounds (boundary conditions). In this thesis, we study these
solutions in bosonic open string field theory using the level truncation
approach, which is a numerical approach where the string field is truncated to
a finite number of degrees of freedom.
  We start with a review of the theoretical background and numerical methods
which are needed in the level truncation approach and then we discuss solutions
on several different backgrounds. First, we discuss universal solutions, which
do not depend on the open string background, then we analyze solutions of the
free boson theory compactified on a circle or on a torus, then marginal
solutions in three different approaches and finally solutions in theories which
include the A-series of Virasoro minimal models. In addition to known D-branes,
we find so-called exotic solutions which potentially describe yet unknown
boundary states.
  This paper is based on my doctoral thesis submitted to the Faculty of
Mathematics and Physics at Charles University in Prague.
