This study sought to redress the lack of proper diagnostic assessment instrument in language instruction, and L2 reading development. More specifically, this dissertation investigated the diagnostic and developmental potentialities of both interventionist (computerized format), and interactionist approaches of DA, to capture L2 reading difficulties in the IELTS context and explore the test-takers’ reading potentialities. This study has two main parts and took place over five developmental phases. Part one explored the capacity of interactionist DA, as a diagnostic tool to investigate the pattern of reading difficulties among the participants as well as the cognitive and linguistic sources of the reading difficulties. To this end, in total, 24 intermediate L2 learners/IELTS candidates were invited to part one of the study through purposive sampling. The interactions were video/audio recorded and transcribed verbatim. Then, the researcher classified patterns of learners’ difficulties and the linguistic and cognitive sources of them. The qualitative analysis revealed that the learners encountered five main difficulties including 1) locating specific information, (2) interpretation of words or phrases within the text, (3) discriminating the main idea from supporting details, (4) inference making, and (5) interpretation of the writer's intention and viewpoint. The content analysis of learners' dialogues, also showed four linguistic and cognitive sources for these difficulties namely 1) lexico-grammatical knowledge, 2) working memory, 3) background knowledge, and 4) reading speed. Part two explored the role of computerized DA for in-depth understanding of the IELTS test-takers’ weaknesses and strengths and exploring the individual test-taker’s reading potential. To this end, a web-based computer program was exclusively designed for this study then, based on the results of two pilot studies some modifications were applied. 65 intermediate IELTS test-takers (scored 5/5-6 in
