The intended readership of this paper is organizations looking to implement very large scale identification systems (e.g. national scale systems which may run to many millions of individuals). Many of the lessons and issues identified will also be useful for organizations looking to develop more general systems based on biometric technology. This paper describes a systematic approach to testing based on lessons learnt from a case study of large scale testing of biometric systems. This approach will enable the performance of the proposed biometric matching system to be characterized to ensure that it is ‘fit for purpose’ and that the benefits outlined in justifying the system can be met.