Patient reported outcomes (PROs) are a measurement based on a report that comes directly from the patient about the status of their health condition without amendment or interpretation of the patient's response by a clinician or anyone else. Valid and reliable PRO instruments are able to provide a standardized, quantifiable measure of treatment benefit, upon which the outcomes of interventions and treatment effect from the patient's perspective can be judged. In some instances, PROs provide the best evidence of a treatment's effectiveness, for example when evaluating treatments for pain, gastrointestinal and urological symptoms, or psychological well-being.This book aims to provide an overview of PRO applications, methodology and validation in order to help reader approach this measurement that are playing an increasingly central role in drug development decision making.