Many countries are now deploying online testing solutions for their terminal post-primary exams e.g. Ireland, New Zealand. These Technology-Based Assessments (TBAs) use items that employ a broad array of interactive, dynamic or static stimuli e.g. simulations, animations, text-image. Although it is assumed that these features can make TBAs more authentic and effective, their impact on test-taker performance and behaviour has yet to be fully clarified. This research investigated the extent to which the use of different multimedia stimuli can affect test-taker performance and behaviour using a mixed methods approach. Guided by four main research questions, an experiment was conducted with 251 Irish post-primary students using an animated and text-image version of the same TBA of scientific literacy. Eye movement and interview data were also collected from subsets of these students (n=32 and n=12 respectively) to determine how differing multimedia stimuli can affect test-taker attentional behaviour. A second study involving 24 test-takers completing a series of simulation-type items was also undertaken. Eye movement, interview and test-score data were gathered to provide insight into test-taker engagement with these items. The results indicated that, overall, there was no significant difference in test-taker performance when identical items used animated or text-image stimuli. However, items with dynamic stimuli often had higher discrimination indices indicating that these items were better at distinguishing between those with high and low levels of knowledge. Eye movement data also revealed that dynamic item stimuli encouraged longer average fixation durations on the response area of an item. An examination of the data relating to test-taker performance and behaviour for simulation-type items found that test-takers developed more efficient search strategies as their familiarity with this item type increased. The data also showed that there was a weak to moderate relationship between task performance and time-to-first-fixation on relevant information. The implications of these and other findings, as well as recommendations for policy, practice, and future research are discussed in the final chapter of this thesis.