Recently, researchers have increasingly focused on understanding commonalities in psychological processes underlying human suffering and psychopathology \[1,2\]. This has been driven by multiple factors, namely: high comorbidity among mental health problems \[e.g., 3,4\]; evidence for within-disorder and between-disorder prediction and cascading effects throughout life \[e.g., 5,6\]; data suggesting that disorder-specific interventions produce improvements in comorbid disorders \[e.g., 7,8\]; established efficacy of transdiagnostic interventions in multiple disorders \[e.g., 9,10\]; and their equivalent efficacy to diagnosis-specific interventions \[11\]. Taken together, research indicates the presence of shared mechanisms, highlighting the need for treatments to target broader processes. Accordingly, transdiagnostic approaches have emerged as promising frameworks, as they may better reflect the complexity and dimensionality of the human experience and more accurately represent the reality of mental health problems.

Acceptance and Commitment Therapy (ACT) is a transdiagnostic approach to behavior change that proposes Psychological Inflexibility (PI) as the root of human suffering in general, and of mental health disorders in particular \[12\]. PI is defined as rigid attempts to control, alter or minimize unpleasant internal experiences at the expense of the ability to persist and/or change behavior to pursue chosen values. PI steams from six interrelated processes: Cognitive Fusion, Experiential Avoidance, Attachment to the Conceptualized Self, Dominance of the Conceptualized Past/Feared Future, Lack of Values Clarity and Inaction, Impulsivity or Avoidant Persistence \[13\]. ACT aims to reverse PI processes by cultivating Psychological Flexibility (PF) which refers to the ability to be in contact with the present moment regardless of unpleasant internal experiences while persisting in value-guided behaviors \[14\]. PF entails six interrelated processes, opposing each PI processes: Cognitive Defusion, Acceptance, Self as Context, Contact with the Present Moment, Values, and Committed Action \[12\].

Evidence supports ACT's efficacy in adults with various disorders \[e.g.,15, 16\] as well as the role of PI/PF components as mechanisms of change following ACT \[17\]. Some studies support that role in adolescents' mental health \[18,19\] and promising results suggest ACT's efficacy with this population \[e.g.,20\]. However, most studies did not consider all PI/PF processes \[e.g., 21\] and there is a scarcity of methodologically robust designs (e.g., Randomized Controlled Trials; RCTs) investigating ACT interventions in adolescents \[e.g.,10\]. Because most studies did not include extended follow-ups and adolescence is marked by significant and rapid psychological changes \[22\], ACT's utility to this age group has not been fully assessed. This seems worrisome considering that between 10 to 20% of adolescents experience mental health problems \[23\]. In youth, anxiety disorders are the most common disorders \[24\]. Particularly, Social Anxiety Disorder (SAD) and Generalized Anxiety Disorder (GAD) present significant prevalence rates in adolescents \[4,25\] both typically presenting a chronic course that may evolve into other mental health disorders in adulthood \[6,26\]. ACT has been proven effective for SAD and GAD treatment in adults \[27,28\]. Preliminary findings point to ACT being efficacious for treating anxiety in adolescence \[23\]. However, few studies examined ACT's efficacy for adolescents' SAD \[20,29\], and only one included adolescents with GAD \[20\]. Research on the efficacy of ACT to adolescents' SAD and GAD is largely missing, and mechanisms underlying change have not been reported. Increasing the understanding of the common mechanisms underlying mental health problems in adolescents, and how these mechanisms can be used to sustain efficacious psychological interventions, is a crucial research concern.

Thus, this project intends to amplify the transdiagnostic application of ACT to adolescents presenting SAD and GAD. A RCT will be conducted to explore the efficacy and processes of change of ACT for SAD or GAD, considering adolescents' anxiety symptoms and flourishing as outcomes. The research team will adapt, implement, and investigate the efficacy of an online delivered (videoconference) ACT Intervention to adolescents presenting SAD or GAD via: : 1. Changes in primary (i.e., anxiety symptoms) and secondary (i.e., flourishing and PI/PF processes) outcomes following intervention - significant improvements are expected at post-intervention only in the intervention groups (i.e., SAD intervention and GAD intervention groups), in comparison with a clinical control group; 2. Examining the stability of change over time (i.e., 3- and 6-months follow-up) - improvements are expected to maintain; 3. Comparing the efficacy of the intervention between both intervention groups - Similar effects on outcome measures for both clinical groups are expected; 4. Investigating mechanisms of change following intervention in both intervention groups - Similar findings in both intervention groups are expected, with changes in PI/PF predicting changes in outcome variables.

All procedures involved in this project (described elsewhere in this form) were approved by the Ethics Committee of the Faculty of Psychology and Educational Sciences, University of Coimbra and the General Directorate of Education authorized the data collection protocol to be implemented in school contexts. Informed consent from adolescents and their parents/legal guardians will be required for all potential participants prior to any data collection. Adolescents and their parents/legal guardians will be informed that the participation is voluntary and that they can decline to participate at any time during the project without any negative consequence. Moreover, they will be informed that the confidentiality of responses will be assured in all moments.