Following the recent interest in the relationship between nutrition and depression, in exploring prevention and treatment strategies for mental health through dietary interventions, this thesis investigates the acute and chronic benefits of wild blueberry flavonoid supplementation on depressive symptoms and cognition in young adults (12-25 year olds). Study 1, a systematic review, revealed that diets containing higher amount of fruits and vegetables have been shown to be inversely associated with symptoms of depression in children, young and older adults. Several micronutrients such as vitamins and minerals present in fruits and vegetables have been investigated regarding their effects on depressive symptoms. However, research on the effects of consumption of flavonoids, that have significant positive effects on cognitive functioning and are abundantly present in fruits and vegetables, on depressive symptoms has been limited and is virtually non-existent in young adults. Given the strong evidence showing the association between cognition and depression, we hypothesised that the beneficial effects of fruit and vegetable rich diets to alleviate symptoms of depression may be via flavonoid related improvement in cognition. However, the direct effects of flavonoids on mood has been unexplored. The collective studies in this thesis attempts to fill this gap in the literature. Study 2 revealed that in 11-17 year olds (N = 77) there was no association between diet quality and depressive symptoms, however the amount of fruits and vegetables consumed was significantly lower than the recommended amounts. Further, more than half the adolescents consumed empty calories above the recommended amount, and none achieved the suggested ratio of carbohydrate to protein to fat in their diet. This in addition to the findings from the systematic review provided a rationale to explore the effects of flavonoids on symptoms of depression and cognitive ability. The acute (2 hours) psychological and cognitive effects of flavonoids were investigated across 2 double-blinded placebo-controlled crossover experiments (Study 3 and 4) in young adults (18-25-year-olds). Study 3 was a crossover study in 21 young adults. No intervention related improvement in cognition or negative affect was observed however, there was a significant improvement in positive affect after the consumption of wild blueberry drinks in Study 3. However, in this study participants’ baseline mood was not accounted for, therefore, the study was repeated (Study 4) in 33 young adults who were screened for depressive symptoms at baseline. No significant influence of flavonoid rich blueberry drink was observed on 12 positive and negative affect or on any of the cognitive measures in healthy young adults or those with elevated symptoms of depression. This comparison requires replication due to the sample size of those with depressive symptoms being small (n=12). Study 5, last in the series of studies is a randomised double-blinded placebo-controlled trial that investigated the effects of 4-week daily wild blueberry (253mg anthocyanins) intervention on transient and chronic mood and cognition in young adults (12-18 year olds). No significant beneficial effect of wild blueberry intervention was observed on transient mood and cognition after 2 and 4 weeks of the intervention. However, participants reported significantly fewer depression symptoms on the Mood and Feeling Questionnaire, after 4 weeks of daily wild blueberry supplementation. The work in this thesis demonstrates that there is low consumption of fruits and vegetables (therefore, low flavonoid intake) in young adults and that, the administration of a …