Fragile X Syndrome (FXS) is a rare genetic syndrome, caused by a mutation in the FMR1 gene located on the X chromosome. It is considered the leading hereditary cause of intellectual disability (ID) and the primary cause of Autism Spectrum Disorder (ASD) due to a single gene-mutation. Many individuals with FXS exhibit symptoms overlapping with those of ASD, including difficulties in social-communication skills, challenges in peer relationships, restricted and repetitive behaviors/interests and deficits in adaptive functioning. Both in ASD and FXS, individuals with greater deficits in executive functions, socio-pragmatic, and socio-relational skills also demonstrate lower adaptive functioning and, consequently, reduced autonomy/independence throughout the life course and greater severity of the disorder. Among empirically validated treatments recommended by National and International Guidelines for the treatment of ASD, cognitive-behavioral and psychosocial interventions have been shown to improve some aspects of ASD, such as core symptoms, emotional-behavioral disturbances, adaptive skills, and quality of life. Currently, it appears that cognitive-behavioral therapies, which include psychoeducation programs, are particularly appropriate for ASD, with greater efficacy for group interventions compared to individual ones. Regarding FXS, despite the well-established knowledge of the cognitive-behavioral phenotype and the clear need for scientifically validated programs, research on intervention strategies remains quite limited. Considering the similarities between ASD and FXS and the need for standardized interventions, the present research project aims to conduct an RCT to evaluate the feasibility of Cooperative Group Therapy (CGT) in two different groups of adolescents with ASD and FXS. The decision to target the intervention to adolescents is due to the few clinical studies on this age group, which is a crucial target since, in FXS, there is often a plateau or reversal of intellectual and adaptive development after the age of 10, and in adolescents with ASD, the development and complexity of social, pragmatic skills, and executive functions are crucial for good adaptive functioning and a basic quality of life. Te main hypothesis is that CGT could contribute to the reduction of severity illness and in the enhancement of socio-communicative skills.