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Emotion regulation difficulty is commonly experienced by autistic adults and has been explored as a transdiagnostic mechanism contributing to anxiety and depression in autistic adults. Previous research has found that emotion regulation strategies and autonomic nervous system (ANS) responses involved in emotion regulation differ between autistic and non-autistic individuals. These findings highlight the role of the ANS as a measure of emotion regulation; however, this has not been studied in an autistic adult sample. The current study examined ANS activity, as measured by respiratory sinus arrhythmia (RSA), root mean square successive differences (rMSSD), and skin conductance level (SCL), and subjective ratings of valence and arousal at rest and while viewing emotional film clips and engaging in emotion regulation in autistic adults. The study consisted of 31 autistic and 31 non-autistic adults, matched on age, intellectual ability, sex, and race/ethnicity. Participants also self-reported their daily emotion regulation use via the Emotion Regulation Questionnaire. Results demonstrated group differences in baseline RSA and rMSSD, self-reported valence of viewing neutral and negative stimuli, and cognitive reappraisal used in daily life. There was also increased SCL and self-reported arousal and decreased self-reported valence to negative stimuli across the whole sample. Exploratory results also showed that SCL and self-reported arousal to negative stimuli were correlated for autistic participants, and that emotion regulation changed physiological and subjective experiences of negative emotion across the whole sample. Supplementary analyses explored the covarying role of co-occurring depression and anxiety and antidepressant use, and showed that these covariates may explain variance in some physiological and self-reported responses whereas in other models, they may suppress these changes. This study extends previous research on emotion regulation in daily life of autistic adults and is the first investigation that demonstrates how an objective measure, such as correlates of ANS, provides additional insight in emotion regulation in autistic adults beyond subjective ratings. This study is important insofar as it shows how emotion regulation strategies manifest in autistic adults and its implications for informing the use of emotion regulation treatment intervention. / Doctor of Philosophy / Autistic adults struggle to regulate their emotions which may contribute to their anxiety and depression. It is previously known that autistic people emotionally regulate differently than non-autistic people, and that their autonomic nervous system (ANS) reacts differently too. These differences may lead to information about how the ANS may measure emotion regulation, yet no one has ever studied this in autistic adults. Therefore, the following study examined ANS activity, as measured by respiratory sinus arrhythmia (RSA), root mean square successive differences (rMSSD), and skin conductance level (SCL), and self-reported ratings of positivity and excitement at rest and while watching emotional film clips and regulating their emotional responses in autistic adults. The study included 31 autistic and 31 non-autistic adults, who had similar age, intellectual ability, sex, and race/ethnicity. Participants also self-reported how often they emotionally regulated, specifically by changing their thoughts ("reappraisal") and keeping a blank face ("suppression") in their daily life using the Emotion Regulation Questionnaire. Based on this study, autistic adults had lower levels of RSA and rMSSD when they were watching a baseline video of swimming fish. They also rated lower levels of positivity when watching both neutral and negative themed film clips and shared that they used reappraisal less frequently in daily life than non-autistic adults. Everyone in the study showed increased SCL and rated lower levels of positivity when watching negative themed film clips. Also, autistic adults with higher SCL also felt greater levels of excitement when watching negative film clips. For everyone, regulating emotional experiences changed the body's responses (i.e., RSA, rMSSD, and SCL) and experiences of positivity and excitement when watching negative themed film clips. However, since having depression and anxiety and taking antidepressants may affect how participants emotionally regulate, additional analyses also explored the impact of these factors; these extra analyses found that some of the previously reported findings may be because of depression, anxiety, and antidepressants, rather than being autistic. Overall, this is the first study to explore the relationship between emotion regulation and autism in adults by looking at the ANS. It is important because it shows how autistic people emotionally regulate which can help experts design mental health treatments.