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Bipolar disorder is characterized by episodes of mania and depression and serious suicidal risks. Recent studies reported high mental imagery susceptibility (general use of imagery in daily life and emotional impact of prospective imagery) in euthymic bipolar patients. This thesis aims to: a) replicate these findings in patients at different phases of bipolar disorder and with varying degrees of bipolarity, and b) explore how mental imagery susceptibility, ruminative processing, and behavioural approach system (BAS) sensitivity interact to amplify mood symptoms. Chapter 1 provides an overview of current theories of mood amplification and recurrence in bipolar disorders. Chapter 2 details the local validation of scales used in the thesis. Chapter 3 (Study 1) investigated whether mental imagery susceptibility, positive rumination and BAS sensitivity were elevated in remitted bipolar I disorder compared with major depressive disorder and non-psychiatric controls. Results suggested that these cognitive variables were elevated in remitted bipolar I disorder. Positive rumination also interacted with positive prospective images to predict bipolarity. Chapter 4 (Study 2) found that these cognitive variables were elevated in bipolar I disorder during manic and euthymic phases, compared to major depression. Further, the number of positive prospective images predicted recovery status and manic symptom severity. Chapters 5, 6 and 7 report that, compared with people without bipolar spectrum conditions, these cognitive characteristics were elevated in sub-threshold bipolar disorder (Study 3), individuals with high bipolar risks based on a behavioural paradigm (Study 4), and individuals with high familial risk (Study 5). Studies 3-5 confirmed that positive and negative prospective images interacted with rumination to amplify hypomanic and depressive symptoms respectively. Chapter 8 (Study 6) showed that suicidal flash-forwards function as a psychological escape from perceived entrapment and defeat in suicidality. Based on these findings, Chapter 9 proposes novel imagery-based techniques for targeting problematic imagery in bipolar disorders.