This chapter discusses considerations for designing, conducting and analysing PRO mode equivalence studies to the highest standards. The background for this article is the increasing use of PRO measures both generally in health research and in pharmaceutical trials. Increasingly electronic versions of PROs (ePROs) are being used for the reasons discussed overleaf. ePRO versions include telephone based interactive voice response systems (IVR), handheld personal digital assistant (PDA) computers and touch screen tablets. As a result of the increasing use of PROs, in 2009 the Food and Drug Administration (FDA) in the United States issued guidance on the use of PROs in regulatory submissions; a short summary based upon the draft of this guidance has also been published. The FDA PRO Guidance states that 'when a PRO instrument is modified, sponsors generally should provide evidence to confirm the new instrument's adequacy'.