Non-small cell lung cancer (NSCLC) is a highly prevalent cancer type with poor prognosis. Nevertheless, a relevant proportion of patients has an extended survival nowadays due to improvements in treatment. Some small studies suggest that these patients have a decreased physical activity (i.e. all activities that a person performs during a day) and decreased physical fitness (i.e. the ability to perform activities). This is thought to negatively impact quality of life and longevity. This project aims to investigate the impact of treatment for NSCLC (surgery, radiotherapy, chemotherapy, immunotherapy, targeted molecular therapy) on physical activity, physical fitness and its relationship with quality of life and symptoms across disease stages. With this aim, we will conduct an observational study and follow patients from diagnosis through the treatment period. This study will be performed as a multicenter study in collaboration with UHasselt. These results will provide insight into which non-pharmacologic treatments patients might need.