This paper presents a theoretical and experimental investigation of tube-bulge-forming. The study is concerned with bulging deformations together with the limiting strains and the factors affecting instability. Theoretical analysis considers bulging of open thin-walled tuhes made of strain-hardening material. A simple approach is adopted in which the bulge profile shape is assumed to be elliptical and the ellipticity ratio is obtained by calculating the constant pressure required to retain this assumed profile shape 4 Results obtained according to this assumption and the assumption of circular profile shape when compared with bulging experiments of commercially pure aluminium tubes, reveal that the actual profile shape approaches an elliptical arc rather than being circular. Limiting strains during tube bulging have also been investigated for long tubes subjected to an internal hydrostatic pressure together with an external axial force. A strain instability criterion which takes into consideration the inevitable geometrical defects in the tube-wall is developed. The results revealed that small eccentricities produce a substantial decrease in the amount of deformation sustained by the tube at instability conditions. Moreover, the sensitivity to such defects increases for tubes made of low strain-hardening materials. The developed strain instability criterion has been subjected to an experimental verification where commercially pure aluminium tubes have been bulged to fracture. Experimental results are found to be in good agreement with theory, thus justifying the validity of this instability criterion and the resulting limit strains.