This document has been produced to complement the Measurement Good Practice Guide No 3 which describes current best UK practice for measuring hot flow stress in metallic materials using Hot Axisymmetric Compression (HAC). This Guide is applicable to hot (isothermal) plane strain compression (PSC) tests at medium to high rates of strain at deformation temperatures below the solidus. Technical input to the document has been provided by a steering group comprising academic researchers, representatives of industrial users and producers of a wide range of engineering materials. An experimental programme was conducted during the preparation of this document to underpin the procedures in this guide.