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.