Introduction: Adults with autism spectrum disorders (ASD) are at heightened risk for several comorbid mental health conditions. However, the prevalence of common co-occurring difficulties such as anxiety and depression and associated risk factors are poorly understood. Aim: The aim of this thesis was to both quantify the prevalence of anxiety and depression in adults with ASD and to investigate how internalising (anxiety and depression) and externalising (conduct problems) symptoms in adulthood are related longitudinally to symptoms in childhood, quantifying any additional impact of exposure to adverse life events and poor parental mental health. Method: The prevalence of anxiety and depression was estimated by conducting a systematic review and meta-analysis consisting of 36 studies, including 30 studies measuring anxiety (n=26,070) and 29 measuring depression (n=26,117). The empirical study included 115 young adults with ASD who were assessed at three time-points (at 12,15 and 23 years of age) on measures of internalizing and externalizing symptoms. Structural Equation Modelling was used to investigate the impact of adverse life events and parental mental health on internalizing and externalizing symptoms. Results: The estimated current and lifetime prevalence for anxiety and depression in adults with ASD was 27% and 42%, and 23% and 37%, respectively. Results of the empirical study indicated that internalizing and externalizing symptoms in young adults with ASD are significantly related to exposure to adverse life events and mental health symptoms in childhood and adolescence. Additionally, poor parental mental health and wellbeing was found to predict a higher frequency of externalising problems but did not moderate the impact of adverse life events. Discussion: The results of this thesis suggest that anxiety and depression are highly prevalent in adults with ASD and that symptom severity in childhood and adolescence, exposure to life events and poorer parent mental health are all independent predictors of symptom severity.