This book explores the appropriateness of Item Response Theory (IRT) in language testing. It investigates the dimensionality of the reading tests of the Cambridge First Certificate of English Test (FCE) and the Test of English as a Foreign Language (TOEFL), and the relative fit of 1, 2, 3 parameter IRT models in which the Rasch model is closely examined. Finding that the Rasch model fails to provide an adequate fit for the data, the study recommends that its predominant use in language testing be re-evaluated. Moreover, the 2 and 3 parameter models fit the data much better than the Rasch model. Finally, it shows that moderate departures from unidimensionality do not necessarily lead to an unacceptable model fit, nor does the use of IRT in test development guarantee that the unidimensionality assumption will be satisfied.