Conformal boundary conditions correspond to classical backgrounds in open string field theory, and consistent bulk theories are believed to correspond to backgrounds in closed string field theory. In the first part of the talk I will focus on the BCFT case, show how one can embed a generic BCFT as part of the open string background and calculate properties of other BCFT's with the same bulk. One can use this approach numerically to learn about hitherto unknown boundary conditions, or analytically to calculate properties of theories related by nearly marginal deformations. I will present a simple general next-to-leading order conformal-perturbation-theory formula for the g-function. In the second part of the talk I will discuss the closed string analog.