The invariant imbedding theory for the monoenergetic neutron transport equation in slab geometry is determined in terms of reflection, transmission, and escape functions. The application of this theory to escape probabilities, blackness coefficients, and critical size determinations is shown along with representative numerical evaluations. The high accuracy of the calculations and the fact that the angular dependence is always obtainable make the invariant imbedding method a reasonable approach to many problems.