Assessment is one of the methods used for evaluation of the learning outputs. Nowadays, use of adaptive assessment systems estimating ability level and abilities of the students is becoming widespread instead of traditional assessment systems. Adaptive assessment system evaluates students not only according to their marks that they take in test exams but also according to their ability levels. In this study, we dealt with theoretical background of the adaptive assessment systems. Study covers the structure, characteristics and architecture of the computerized adaptive test systems in detail. It dwells on the Item Response theory which is used for improving computerized adaptive test systems and the models with which this theory is used. Besides, processing steps which are required to realize a computerized adaptive test system and algorithms used for the validation of the test systems are introduced in detail. Briefly, study introduces the structure and the improvement processes of the computerized adaptive testing systems which can be integrated to distance education software. It is possible to say that integration of these systems into distant education software allows individualized learning and individualized assessment studies to be carried out successively in distant education software as well.