Cactus spines from 21 species out of two subfamilies, six tribes and 14 genera were examined by electron microscopy to determine their structural properties. In addition, the spines were tested for their bending stiffness and the results compared with the structural properties. All the spines observed were built of longitudinally oriented fibrous structures. However, they clearly showed two different construction principles. Some spines were built of sheets, placed in layers with different fibre orientation and showing a folded appearance in cross section. Others were built from pillar-like structures, where the main building blocks consisted of longitudinally oriented strands of fibres. Where those spines with a folded structure were mainly (8 out of 11) curved, those with a pillar structure were straight (9 out of 10). The range of bending stiffness, in the spines examined, ranged from 0.007 GPa up to 12.7 GPa. These values were compared to those for common materials, from rubber to hard wood. These findings could be used for taxonomy or could serve as templates for man-made composite structures.