Chemotherapy and adjuvant endocrine treatment for breast cancer has been associated with varying degrees of cognitive dysfunction, with 15-50% of women reported to experience subtle cognitive decline. While these treatments may have direct adverse consequences on neurological functioning, cancer diagnosis and treatment is also associated with many health and psychosocial factors that may decrease performance on neuropsychological tests. However, despite a growing body of literature on affected cognitive domains and observable neurological changes after chemotherapy, there has not been a thorough investigation into potentially important psychosocial and physical health mechanisms that may underlie the observed cognitive dysfunction. Therefore, the primary aim of this thesis was to evaluate the relationship between health/ treatment, psychosocial, and endocrine-related factors and cognitive dysfunction after breast cancer treatment. In addition, a smaller secondary aim was to assess the appropriateness of different methods of individual change. Chapter 1 provides a brief overview of the structure and content of the thesis. Chpaters 2 and 3 are review papers that evaluate whether there is evidence that variations in psychosocial adjustment, health and treatment factors result in cognitive changes after chemotherapy. Based on previous research, the mechanisms evaluated are endocrine-related changes (use of adjuvant endocrine treatment and chemotherapy-induced menopause); chemotherapy-induced anaemia; depression; anxiety; fatigue; quality of life; and other treatment factors (e.g. treatment duration, time since treatment, tumour stage, use of concomitant medications and co-morbid medical conditions). The impact of confounding variables such as age of participants, level of baseline functioning and methodological limitations are also considered. These two chapters have been published (refer to Appendix A for a complete list of presentations and publications arising from this thesis). The fourth and fifth chapters are methodological in nature. Chapter 4 describes methods, while Chapter 5 is a brief paper (under review) which examines methodological considerations regarding analysis of individual change in neuropsychological performance over time and across domains for women undergoing treatment for breast cancer. The sixth and seventh chapters involve empirical analyses of the data collected as part of the Cognition in Breast Cancer (CBC) study, a longitudinal study examining the causes of variation in cognitive functioning, health and well-being in women up to 2 years post-chemotherapy. Chapter 6 was an experimental study designed to investigate the acute effects of psychosocial mechanisms on cognitive functioning after chemotherapy in a sample of 157 breast cancer patients. Many of the methodological limitations identified in the review studies were addressed and the neuropsychological performance of two groups was compared, namely recently diagnosed breast cancer patients scheduled for chemotherapy (n = 136) or other forms of treatment (n = 21). Participants were assessed prior to commencing treatment and approximately one month post completion of chemotherapy (or 6 months after the first assessment). Individual cognitive impairment was examined using the Reliable Change Index, while Pearson correlations were utilised in order to investigate the effect of psychosocial and health factors on cognitive change. The results indicated that decline in haemoglobin levels and increased anxiety over the course of chemotherapy significantly predicted impairment in multiple cognitive measures, while change in specific cognitive measures was significantly associated with baseline measures of fatigue, depression and functional well-being. The impact of these findings on rehabilitation strategies for women after chemotherapy was discussed. Chapter 7 investigated whether endocrine-related changes, namely chemotherapy-induced menopause and adjuvant endocrine treatment, resulted in increased cognitive dysfunction. One hundred and thirty-six breast cancer patients were assessed using a comprehensive neuropsychological assessment over three time-points, namely pre-chemotherapy, one month and six months post chemotherapy (or at similar time-points). Linear mixed models evaluated the effects of these two factors, with little evidence found to suggest that endocrine-related factors contribute to cognitive dysfunction in breast cancer patients. Chapter 8 comprises a brief summary and overview of the entire thesis and offers overarching conclusions, strengths and weaknesses, and directions for future research. The findings of the present investigations attempt to elucidate the contributions of potentially important psychosocial and health/ treatment-related mechanisms for cognitive dysfunction after breast cancer treatment. While there was little evidence to suggest endocrine-related changes impacted on cognition, the findings linking chemotherapy-induced anaemia and baseline psychosocial measures may play an important role in identifying and treating at-risk individuals. These findings have potential research implications for the ways data is collected, analysed and presented in empirical research as well as clinical ramifications for how women are affected cognitively as well as psychologically by treatment for breast cancer.