We study the propagation of X-rays in very narrow slits or waveguides. The onset of waveguiding effects is estimated analytically and quantified by finite-difference field calculations. A critical thickness is given below which waveguiding effects have to be taken into account. Finally, we consider the transmission of the smallest possible slits i.e. planar single-mode waveguides. (c) 2006 Elsevier B.V. All rights reserved.