Performance assessment is increasingly important in education and training, with its focus on the ever growing requirement to demonstrate ability in practical communication skills. Yet many fundamental problems in performance based assessment still remain. This book provides a critical examination of the practice and theory behind performance based assessment in the context of second language learning. It also offers the first comprehensive presentation of a new measurement approach - Rasch Measurement, which enables investigation of aspects of performance setting (rater and task characteristics for example) to determine their effect on the scores awarded to candidates and to manage this variability fairly.