Over the past decade, cognitive research using Transcranial Direct Current Stimulation (tDCS) sparked enormous interest in its potential to safely, easily, and affordably boost cognitive function in healthy and clinical populations. However, cognitive effects from tDCS have been mixed, especially as it pertains to enhancing higher-level cognitive functions such as episodic and working memory. Recent work in our own lab (Wong et al. 2018) suggested that time-of-day may be a critical factor in tDCS effects in younger adults, by examining whether these effects replicate and also extend to older adults who have known changes in circadian preferences. To more definitively address these questions, we conducted the first NIH-funded clinical trial of tDCS in younger and older adults at different times of the day, with considerably larger sample sizes than are typical of this literature. Specifically, we used a double-blind, sham-control, between-subjects stimulation design, administering anodal tDCS to left dorsolateral prefrontal cortex (dlPFC) prior to episodic memory retrieval and working memory tasks in both the AM and PM. We also investigated the effects of left posterior parietal cortex (PPC) stimulation in younger adults. In Chapter 1, we report the effects of tDCs on our two memory measures. Results demonstrated the potential role of task difficulty in eliciting stimulation-related boosts in performance as it relates to the effects of anodal left dlPFC stimulation on working memory performance, but not episodic memory retrieval. In Chapter 2, we report the effects of circadian preference and sleep quality data and its impact on tDCS effects in memory performance. While the optimal methods for obtaining reliable tDCS results continue to be elusive, these results point to the possibility that tDCS may be conditional on sleep quality -- a variable never before investigated in this literature. Indeed, adjusting our models to account for actigraphy-derived sleep quality underscores the potential of tDCs to boost episodic memory, as well as mitigate declines in memory performance associated with poor sleep quality. Overall, these results provide some support for the hypothesis that tDCS benefits performance “when needed”, but given the historically tenuous nature of tDCS effects, we emphasize the need for further replication of this potentially important discovery. Finally, in Chapter 3, we report the effects of tDCS expectations on performance. These findings raise serious questions about the validity of within-subjects tDCS designs that are frequent in the literature, and emphasize the potential importance of between-subjects designs, as used in the current study. We conclude by discussing important methodological considerations centered around the importance of rigorous blinding and shamming protocols necessary to achieve more reliable (and replicable) tDCS effects of healthy cognitive function and beyond.