Chronic obstructive pulmonary disease (COPD) is characterized by airflow limitation that is not fully reversible, though a number of pulmonary phenotypes are recognized. These include small airways diseases, chronic bronchitis and bronchiectasis, as well as pulmonary emphysema, which can be further subdivided by the zone of the lung which it affects, and its radiological appearance. In addition COPD is associated with a number of comorbidities, which are found more frequently than would be expected by chance, even after controlling for common etiological factors (such as smoking or steroid use). These comorbid conditions may be responsible for some of the deterioration and de-conditioning seen in COPD, as well as a significant proportion of mortality, and should be sought and managed where clinically appropriate. This review examines the prevalence and clinical features of associated comorbid conditions, including atherosclerosis, cardiac failure, diabetes, osteoporosis, cachexia, gastro-esophageal reflux disease and depression. A brief consideration of their management in COPD is also given. In addition evidence for the concept of pulmonary overspill leading to systemic inflammation, the consequences of systemic inflammation, the possibility of accelerated aging, and of how these concepts could relate to shared genetic risk factors for both comorbidity and pulmonary aspects of COPD is discussed.