This sixth module in the series for preservice elementary teachers develops the following concepts of diagnosis in mathematics: diagnostic techniques and instruments; the identification of specific learner behaviors; the concrete, semi-concrete, and abstract continuum aspect of learning mathematics concepts and skills; Piagetian developmental stages for conservation of number, length, volume, substance, and area; learner error patterns; and remedial procedures. Objectives, prerequisites, and pre- and post-assessment procedures for this module are specified, and sixteen instructional activities to be completed by the preservice teachers are listed. Fifteen reading handouts are included with this module. (DT)