Poisson's ratio is defined strictly on the assumption that the material is homogeneous. In applications of Poisson's ratio to mechanical problems, this assumption can be overlooked. To check whether the assumption applies to monotonic and cyclic loading, we review measurements of Poisson's ratio, or rather the ratio of diametral strain to axial strain. Such measurements are quite rare for cyclic deformation. However, they show that the ratio of diametral strain to axial strain can differ from values expected on the basis of traditional values of Poisson's ratio for elastic and plastic deformation. The technical consequences of this difference are indicated.