Inclusion Criteria:  Pathologically (histologically or cytologically) proven diagnosis of NSCLC  Planned initiation (within the 30 days after study registration) of fractionated (≥15 treatments) thoracic radiotherapy with concurrent chemotherapy. Expected treatment scenarios include:  Definitive treatment of stage III disease (most common) Definitive treatment for locoregional recurrence of early stage disease Definitive treatment of unresectable stage II disease Preoperative treatment of stage III disease Postoperative treatment after incomplete resection (uncommon) Radical treatment for a patient with advanced/metastatic disease with the goal of achieving long-term thoracic disease control (e.g., oligometastatic disease where metastases are addressed with resection or stereotactic radiotherapy) ECOG Performance Status 0-3 within 30 days prior to registration Age ≥ 18 Ambulates independently or with a cane (use of a walker not permitted) Patients who already use wearable devices and/or smartphones that monitor physical activity are eligible for this trial. Patients must agree to wear the device provided by the study. The patient or a legally authorized representative must provide study-specific informed consent prior to study entry.  Exclusion Criteria:  Patients with a history of another cancer that is expected to affect the toxicity profile of thoracic chemoradiotherapy for NSCLC Patients receiving concurrent treatment for another cancer that is expected to affect the toxicity profile of thoracic chemoradiotherapy Patients receiving thoracic radiotherapy without concurrent chemotherapy are ineligible. Patients receiving concurrent chemotherapy and another form of systemic therapy (e.g., immunotherapy) are eligible. Patients receiving radiotherapy with concurrent immunotherapy or targeted therapy but without chemotherapy are ineligible