COPD is a prevalent chronic health condition secondary to only heart disease and stroke. A review of the literature reveals whilst smoking is the most common aetiological factor, environmental and occupational pollutions, genetic predisposition, hyper-responsive airways and respiratory infections are additional mechanisms to the development of COPD in susceptible individuals. Assessment of the severity of COPD via spirometry is considered the gold standard in quantifying the degree of respiratory impairment experienced by the client with the condition. However, there are inconsistent parameters of disease severity between each of the international respiratory bodies, which must influence reporting of outcomes. This paper reviews the literature reports of the mechanisms; systemic effects, disease-staging systems used in COPD and the relationship between disease severity and health outcomes.