Autistic youth are more likely to have thoughts of suicide than non-autistic youth, with as many as one in four having thoughts of suicide and almost one in ten attempting suicide. The reasons why autistic youth may have these thoughts are likely different than those for non-autistic youth and researchers are not yet able to easily identify autistic youth who are likely to have thoughts of suicide or attempt suicide. Autistic youth are more likely to report feeling lonely, which is one possible reason why they may be more likely to attempt suicide. However, there is a need for more research on loneliness in autistic individuals. Current available information relies on questionnaires that ask autistic youth to share how they felt over several months and may be difficult to answer accurately. Therefore, we propose to examine how autistic adolescents experience loneliness by asking them to report on it in the moment using their smartphones. We will also collect data on their thoughts of suicide and their difficulty identifying and expressing emotions to see if this difficulty is related to loneliness and thoughts of suicide. Results of this study can provide several clinically meaningful outcomes for autistic individuals. First, this study can help improve understanding of loneliness and help identify autistic adolescents who may be likely to experience loneliness and thoughts of suicide. This can lead to more clinicians being able to detect these youth earlier and offer personalized intervention. Second, findings can help challenge the narrative that autistic individuals are not socially motivated and therefore will not feel lonely even when alone. This holds potential to reduce stigma and improve quality of life for autistic individuals.