Chronic obstructive pulmonary disease (COPD) is a heterogeneous disease that cannot be described by the severity of airflow limitation (forced expiratory volume in the first second of expiration) only. Other measures are needed in clinical practice to assess patients, predict their risk, guide their treatment, and assess their response to it. Over the past few years, there has been a great deal of interest in the identification and validation of biomarkers of potential clinical use in COPD. Here, the authors review some general concepts in the field, discuss currently validated biomarkers in COPD, and speculate on potential future developments.