Many groups have investigated cognitive performance during hyperthermia and dehydration, with few demonstrating convincing and unequivocal influences. Some reports show neither thermalnor hydration-induced influences, others have found improved, whilst some report reduced cognitive performance. This confusion has arisen due to methodological limitations that have resulted in many previous experiments not being optimally designed to evaluate these effects. For instance, few studies have appropriately induced hyperthermia and dehydration, and then clamped these states during the cognitive challenge. Many investigators have used physical exercise to induce these states, yet exercise may independently affect cognitive performance. Furthermore, task difficulty has rarely been controlled across cognitive functions, with the difficulty level for many tasks being too low, whilst inter-task comparisons have often been performed across different levels of difficulty. The former introduces bias, such that only performance decrements can be observed, whilst the latter renders it almost impossible to compare either the baseline data or subsequent changes in cognitive performance during altered thermal and hydration states. As a consequence of these limitations, our understanding of the affects of these stresses upon cognitive performance is less than optimal, and this study was designed to address these design limitations.