Some problems in the measurement of stress-strain curves for thermoplastics are considered and the importance of measuring true stress-strain curves is emphasized. However, an increasing number of suitable curves have now been published of which the large majority have been found to comply with the following Gaussian equation : sigma(true)=Y+G(p)(lambda(2)-1/lambda) where sigma(true) is the true stress, Y is the extrapolated yield stress, G(p) is the strain hardening modulus and lambda is the extension ratio. It is shown that the Considere condition for necking requires that Y/G(p)>3 and the available results support the proposition that this is a necessary condition for necking to occur. High values of G(p) are observed in polymers with an extended chain conformation.