Inclusion Criteria:  1. read the study information and sign informed consent; 2. are 18 years or older; 3. have access to the Internet, to a smartphone, PC or tablet; 4. have sufficient knowledge of German; 5. exceed predefined cut-off scores on one of the social anxiety measures (22 points on the Social Phobia Scale or 33 points on the Social Interaction Anxiety Scale; SPS & SIAS); 6. fulfil the diagnostic criteria of SAD according to the diagnostic telephone interview (the presence of other disorders does not result in exclusion if SAD is the primary diagnosis) 7. in the case of taking psychiatric medication, the treatment is stabilized (over one month)  Exclusion Criteria:  1. they score 2 or higher on the suicide item of the PHQ-9 (Spitzer, 1999) or with active suicidal plans in the diagnostic telephone interview. 2. they have other comorbid psychiatric conditions that might be the main focus of clinical attention (e.g., psychotic symptoms, bipolar disorder, or other severe disorder) 3. they undergo another psychological treatment at the beginning of the study.