The aims, mainly, are to explain the role of AISs in manufacturing companies in rationalizing IDs in general. In addition, it is concerned with: a) determining to what extent manufacturing companies in Egypt rely on the current methods in evaluating their investment projects; b) assessing the extent to which AISs applied in manufacturing companies in Egypt provide a basis for using the current methods in evaluating their investment projects; and c) using the results of this assessment to suggest how the applied AISs might be improved to meet the needs of the process of investment decision-making. I revealed that there is a gap between theory and practice in the area of the role of AISs in rationalizing IDs generally and in Egypt particularly. Some of the gap can be attributed to deficiencies in the theory and some of it can be attributed to practice, especially the shortcomings of AISs. Therefore, accountants, especially those in manufacturing companies in Egypt, need to expand their knowledge and skills beyond the traditional disciplinary remit of accounting theory and practice, and the ID making-process.