This Workshop aims at stimulating multidisciplinary discussions about the challenges related to corpus creation and annotation for social skills behavior analysis. Contributions from computational, psychological and psychometrics perspectives, as well as applications including platforms to share corpora and annotations, are welcomed. The main challenges related to corpus creation include the choice of the best setup and sensors, finding a trade-off between eliciting natural interactions, limiting invasiveness and collecting precise information. The second issue in this context regards the process of annotation. The choice of the type of annotators (experts vs. nonexperts), the type of annotations (automatic vs. manual, continue vs. discrete), the temporal segmentation (windowed vs. holistic) is crucial for a correct measure of the phenomenon of interest and getting significant results. The topics of CATS2021 will have a strong impact on researchers and stakeholders across different disciplines, such as Computer Science, Social Signal Processing, Psychology, Statistics. Leveraging the opportunities offered by such a multidisciplinary environment, the participants could enrich their perspective, strengthen their practices and methodologies and draw together a research roadmap tackling the discussed challenges, which might be taken up in future collaborations.