Anxiety experiences (defined either categorically as a disorder or on continua as symptoms) have been found to be highly prevalent in bipolar disorder (BD) and have been consistently associated with poorer outcomes. Current research in this area has primarily focused on prevalence rates of anxiety disorders and their association to retrospective outcomes. There is a lack of research regarding the psychological processes which may underlie the relationship between anxiety and bipolar mood experiences and current psychological models of BD have generally omitted anxiety in their explanations of mood swings. A qualitative meta-synthesis and semi-structured interviews were employed in this thesis to explore the lived experience of anxiety in BD. A longitudinal analysis of data from a large scale RCT was also conducted to assess a range of categorical and continuous measures of anxiety as predictors of outcome in BD. Finally, experience sampling methodology assessed momentary interactions between anxiety and affect in daily life for individuals with BD and non-clinical controls. Anxiety was found to be intrinsically linked to bipolar mood experiences across methodologies. Subjectively, anxiety was perceived as a trigger to both depressed and manic experiences. Anxiety about relapse due to extreme negative appraisals of mood swings was reported and impacted on several important life domains including quality of life, sleep, relationships and employment. Anxiety was consistently associated with increased depression and reduced functioning across all studies. Anxiety and mania were found to have both positive and negative associations across studies. The continuous measurement of anxiety, rather than categorical, was the most reliable predictor of outcome longitudinally when carefully controlling for a range of extraneous variables. The results provide support for anxiety as an intrinsic experience in BD and encourage the consideration of integrated psychological models and treatment approaches which include anxiety as a core feature of BD.