This article investigates how theological properties of polypropylenes with different molecular structures influence their foaming behavior. The molecular structure of the different polypropylenes is analyzed by size exclusion chromatography coupled with a light scattering detector, and by theological means, such as the molar mass dependence of the zero shear-rate viscosity. The main focus of the theological experiments is laid on the strain hardening and failure behavior of the melts in uniaxial elongational flow. For all linear polypropylenes investigated, a rupture of samples occurred before the maximum strain-accessible with the instrument used-was achieved. For the linear polypropylenes, a growth of the stress to rupture and the elongation at break were found with increasing molar mass, which go along with an increase of the expansion ratio in the foaming experiments. Besides the linear polypropylenes, which do not show strain hardening, several so-called high melt strength polypropylenes were investigated. It was found that the strain hardening of those polypropylenes causes not only a high melt strength, but also a more uniform sample deformation in the elongational experiments, and thus high elongations at break or even no rupture of the samples. Due to the superior homogeneity of deformation, the foams of polypropylenes, which show pronounced strain hardening at the strain rates relevant for foaming, possess higher expansion ratios than linear samples of the same melt strength. Furthermore, it was found that the foaming behavior of the polypropylenes is governed by the elongational behavior of the melts independently of the molecular constitution, which is responsible for it. This means that different molecular structures, which cause a similar elongational behavior of the melt, lead to a comparable foaming behavior. (c) 2006 The Society of Rheology.