We review the Considere construction and discuss its applicability to the prediction of the inception of necking in extensional flow. The behaviour of simple fluid models is used to illustrate the importance of fully specifying the flow conditions. Even for a Newtonian liquid there is a marked difference in the prediction of necking under the application of a constant force and the imposition of a constant rate of strain. (C) 2009 Elsevier Ltd. All rights reserved.