  We discuss D-branes from a conformal field theory point of view. In this
approach, branes are described by boundary states providing sources for closed
string modes, independently of classical notions. The boundary states must
satisfy constraints which fall into two classes: The first consists of gluing
conditions between left- and right-moving Virasoro or further symmetry
generators, whereas the second encompasses non-linear consistency conditions
from world sheet duality, which severely restrict the allowed boundary states.
We exploit these conditions to give explicit formulas for boundary states in
Gepner models, thereby computing excitation spectra of brane configurations.
From the boundary states, brane tensions and RR charges can also be read off
directly.
