In the present study, 90 nm thick conducting tin-doped indium oxide (ITO) layers on polymer substrates are used to study the failure behaviour of brittle layers. In a two-point bending test the resistance of uniform ITO-layers and narrow ITO-lines (10 to 300 μm width) are determined as a function of the applied tensile strain. At a certain strain, the thin layer will crack and the resistance will strongly increase. This characteristic strain is analysed using a large number of samples. Early stages of crack development are studied in the fragmentation test. This paper presents the results of different failure mechanisms of thin brittle layers on the electrical conductivity of ITO-layers. When the strain in the ITO-layer increases, stable cracks of a limited length are initiated at defects in the layer (crack initiation). At the critical strain εp the crack is no longer stable and it will propagate over the whole width of the layer (crack propagation). The experiments show a wide failure strain distribution for narrow ITO-lines. The wide distribution of defects, determining the crack initiation strain, controls the failure of narrow lines. The uniform layers show a narrow failure strain distribution. This is determined by the well-defined crack propagation strain εp.