A homotopy boundary of a finite CW-complex X is the boundary of a thickening of X, that is of a compact manifold of the homotopy type of X. This notion appears naturally in the context of embeddings since the boundary of a tubular neighborhood of the embedded manifold in the ambient manifold is a homotopy boundary of the embedded manifold. The purpose of this talk is to study the rational homotopy type of the homotopy boundaries of a given complex X. In particular, we will see that the rational homotopy type of a large class of boundaries is completely determined by the rational homotopy type of X.