For many years personalization of achievement testing has been impossible in all but the simplest forms. Recently, item response theory (IRT), or latent trait theory, has emerged as a valuable tool which brings far greater flexibility to the process than had previously been possible. The single parameter Rasch Model, a mathematical model developed by George Rasch, has proven particularly useful in a number of instances in which it has been used. These situations range from individual school districts, through consortia of school districts, to governmental agencies. IRT makes possible the measurement of performance in relation to latent traits in a way which is especially appropriate to basic skills achievement testing in the public schools. Recent developments in microcomputer technology and the development of computer programs make it practical for the local school district to develop and calibrate files of items to meet its own testing needs. Calibrated item files are becoming more available for purchase by those potential users who would sooner not develop their own. The transition from item file to finished test is well within the scope of the local user. Available procedures range from the intensively clerical to the highly computerized and are well documented in readily available publications. (JAZ)