Rheological problems are non-homogeneous, aelotropic, non-linear and irreversible. If a start is made with the stress-tensor field, one soon runs into a number of difficulties. The strain tensor is defined for any type of medium-homogeneous, heterogeneous, isotropic, aelotropic and hence should be a suitable vehicle for investigation the yield condition. Again, the classical strain/strain rate, when used, makes the constitutive equation very complex. So the generalized strain measure should be used.