Chronic obstructive lung disease (COPD) is characterized by airflow obstruction that is progressive over many years and is largely irreversible. Advanced COPD is associated with arterial oxygen desaturation leading to a series of complications and, ultimately, decreased survival. Long-term oxygen therapy can improve clinical outcomes in these patients, but the exact target of oxygen saturation that actually translates into improvements is not known. The basis for the work in this proposal is to focus a new approach to measure oxygen desaturation linked to daily activity. Accelerometers are used to measure daily activity and then synchronized with ambulatory oximetry to establish an activity/oxygen-saturation profile for individual patients. The three main objectives of this study are 1) determine the feasibility of AOM as a measurement of the temporal profile of oxygen saturation in patients with chronic lung disease; 2) determine if serial AOM-derived data is reliable and reproducible; and 3) determine thresholds of oxygen desaturation that are associated with different activity profiles