Physical activity is reduced in patients with chronic pulmonary diseases. Activity monitors can measure physical activity objectively and accurately over prolonged periods of time. Research grade and commercially available devices, using accelerometer technology, are being increasingly used in clinical studies. Physical activity levels have been found to have a moderate to strong correlation with important measures such as pulmonary function, exercise capacity, quality of life, and mortality and hospitalizations in patients with COPD, interstitial lung disease, pulmonary arterial hypertension and cystic fibrosis. Their use as a clinical trial end-point and as a tool to augment rehabilitation efforts has also been explored in patients with COPD with variable results. Due to the ease of use, economic viability, widespread availability and good patient compliance, their use in adult and pediatric medicine is expanding. This narrative review summarizes the current evidence of use of activity monitors in COPD, interstitial lung disease, asthma, pulmonary arterial hypertension, cystic fibrosis and lung transplant patients for the purposes of prognostication, monitoring, outcome measures and intervention.