Chronic obstructive pulmonary disease is a major cause of morbidity and mortality worldwide and an important cause of disability. A thorough patient-centered outcome assessment, including not only measures of lung function, exercise capacity and health-related quality of life, but also of functional capacity and performance in activities of daily life, is imperative for a comprehensive management of chronic obstructive pulmonary disease. This American Thoracic Society Seminar Series is devoted to help clinicians substantiate their choice of functional outcome measures in this population. In Part 1 of this two-part Seminar Series, we aim to describe the various domains of functional status, to elucidate terms and key concepts intertwined with functioning, and to demonstrate the clinical relevance of assessing functional capacity in the context of activities of daily living, in agreement with the International Classification of Functioning, Disability and Health. We hope that a better understanding of the various defining components of functional status will be instrumental to healthcare providers to optimize chronic obstructive pulmonary disease evaluation and management, ultimately leading to improved quality of life of patients afflicted by this condition. This first paper also serves as an introduction to Part 2 of this Seminar Series, where the main functional tests available to assess upper and lower body functional capacity of these patients will be discussed.