Dirichlet boundary condition has been previously used in Chap.  10 . This chapter gives an overview of the main boundary conditions and the mathematical expression of linear boudary conditions in 1D geometry. The notion of extrapolation distance and the behavior of flux and currents at the boundary of the system are also presented at the end of the chapter. We only consider convex reactors: thus any neutron escaping from the system will not be able to re-enter.