Small cell lung cancer (SCLC) is an aggressive disease that is characterized by rapid growth and the early development of metastases. Patients typically respond to initial chemotherapy but quickly experience relapse, resulting in a poor long-term outcome. Therapeutic innovations that substantially improve survival have historically been limited, and reliable, predictive biomarkers are lacking. Ongoing research has advanced the understanding of molecular categories and the immunologic microenvironment of SCLC, which in turn has helped improve disease classification and staging. Considering the role of molecular alterations has not yet fully to be defined in the treatment of SCLC, there is an urgent recognition that molecular alterations in the SCLC are important to predict response and survival for novel therapies and ongoing clinical trials. Advances in research have revealed critical information regarding biologic characteristics of the disease, which may lead to the identification of vulnerabilities and the development of new therapies. Further research focused on identifying biomarkers and evaluating innovative therapies will be paramount to improving treatment outcomes for patients with SCLC. In summary, identification of (genetic) biomarkers in SCLC is increasingly essential to perform molecular diagnostics and individualized treatments. This project aims to create a registry of patients with SCCL to further the characterization of molecular alterations and develop (novel) treatments based on the detection.