Flexible adhesives, characterised by low modulus and large extensions to failure, have a long history of use in non-structural applications. However, the advantageous properties of flexible adhesives in sustaining large strains and more evenly distributing peel forces on the bonded substrates are leading to their use for structural joining applications. This is driving the need to improve the understanding of their mechanical properties, which has received little attention, in comparison to structural adhesives. The study of methods for characterising the deformation and failure of flexible adhesives has formed the core of the Flexible Adhesives project of the Performance of Adhesive Joints programme supported by the DTI. This Measurement Good Practice Guide describes methods for characterising the hyperelastic and visco-elastic properties required to predict the mechanical performance of a flexible adhesive layer in a bonded structure. The Guide also discusses some aspects of Finite Element modelling of flexible adhesives.