Our research has focused on developing an Online Diagnostic Assessment System (ODAS) for the first six grades of primary school. The main aims of the current phase of the ODAS project are (1) to establish a system for assessing the progress of students in several dimensions of cognitive development; (2) to explore the possibilities technology-based assessment offers for a better understanding of students´ cognition, especially reasoning of students with learning difficulties; and (3) to develop methods for compensating the diagnosed developmental deficiencies and for accelerating the development of some specific and general skills. Computer games are considered as one of the best means of interventions fostering the development of basic reasoning skills. The work that forms the empirical basis of this chapter can be characterized by four distinguishing features. (1) Designing games is driven by specific educational purposes. The activities of the games correspond to knowledge and skills identified in the assessment frameworks. (2) The games are designed for young children; therefore, they have to meet several specific requirements. (3) Innovative assessment technologies are explored by logging and analyzing metadata, such as keystrokes, mouse movement, head movement, and facial expressions (captured by web camera). (4) Assessment tools intend to monitor both cognitive (e.g., reasoning, speed carrying out operations) and affective (motivation, interest, boredom, fatigue) processes. The chapter summarizes the theoretical (cognitive psychological, educational and information technological) background of the ODAS project and reports the results of a pilot intervention with some general conclusions for further research.