In recent years, numerous studies have investigated the potential contributing role of the autonomic nervous system (ANS) in psychiatric disorders, including autism spectrum disorder (ASD). One of the most commonly adopted physiological parameters for assessing ANS functioning in psycho-physiological research is based on the assessment of heart rate variability (HRV). The heart's pacemaker is controlled by both the sympathetic and the parasympathetic nervous system, which results in the natural variation of the interval between heart beats. One important component of HRV is commonly known as respiratory sinus arrhythmia (RSA). This component is solely mediated by the parasympathetic subsystem through direct projections of the vagus nerve from the brain stem to the heart and is therefore also known as an index of cardiac vagal modulation. The term RSA is adopted since it defines the phenomenon of the relation between heart rate variability and the respiratory cycle in certain circumstances. The clinical importance of RSA has been reported in multiple studies as higher RSA is associated with higher cognitive abilities, effective social behavior, fewer internalizing symptoms (e.g. anxiety and depression) and appropriate emotion regulation. In addition, lower RSA has been linked to conduct problems, trait hostility, anxiety disorders and depression. With respect to children and adolescents with ASD, several studies have investigated the ANS functioning by assessing RSA during various activities (e.g. cognitive tasks, non-social auditory tasks, social cognition tasks and social interaction tasks) as well as during resting situations. While some studies revealed no significant differences in RSA levels between ASD and typically developing (TD) peers, the majority of studies seemed to converge on demonstrating lower levels of RSA in patients with ASD. Furthermore, in these studies, lower RSA levels in ASD were associated with more social problems, internalizing problems, higher levels of anxiety, problems with emotional control, the presence of more severe visual and/or auditory sensory problems and the presence of repetitive and restrictive behaviors. In line with the link between lowered RSA and more psychological and social problems, it has been posited that it would be of great relevance to develop and validate an intervention that specifically targets the up-regulation of RSA-values, or the cardiac vagal modulation, in individuals with ASD . To do so, biofeedback-based interventions have been put forward as these involve the recording of physiological signals (e.g. RSA or skin conductance) which are transformed into visual (or auditory) presentations and subsequently provided to the individual as direct feedback. This visualisation of ongoing bodily sensations would provide an added value for individuals with ASD, as previous studies have reported a lower ability to consciously perceive internal bodily sensations (interoceptive awareness) in this population. Considering that adolescence is a vulnerable period with rapid changes in distinct developmental domains, this population will be targeted during this research project. As previously mentioned, the clinical importance of RSA in both typically developing populations and populations with ASD has already been demonstrated. However, due to methodological differences, these studies are frequently incomparable and/or often lead to conflicting evidence. Therefore, a cross-sectional phase will be conducted first in this research project in which adolescents with and without ASD will be recruited. In this phase, they will perform a stress-inducing test in which physiological, endogenic and behavioral data will be collected. The main focus of this first part is on RSA in which lower RSA-values are hypothesized in adolescents with ASD as compared to their typically developing peers, based on previously mentioned study results. The second part of this research project consists of the evaluation of an RSA-based biofeedback approach for the up-regulation of RSA in adolescents with ASD. In other words, we will explore the potential of this intervention for enhancing cardiac vagal modulation and the influence on behavioral problems related to lower RSA in individuals with ASD as determined in previous research and based on findings during the first part of this study. Importantly, the validation of an RSA biofeedback intervention for adolescents with ASD may open novel avenues for home-based interventions in this population where therapist contact can be limited and integration into the daily routine can be supported using biofeedback apps. In addition, the current worldwide circumstances due to the corona virus disease has provided clinicians with new and useful information regarding their patients with ASD as these patients perceived the increase of home-based interventions as something positive. Therefore, the biofeedback intervention in this research project may have an additional advantage due to its home-based approach. Finally, the promising results of RSA biofeedback interventions in clinical populations, including ASD, are reported in various domains, going from decrements in anxiety and depression symptoms to improvements of social functioning. Therefore, this intervention may be valuable for adolescents with ASD given their vulnerable position in development.