Exacerbations are important events in patients with asthma and chronic obstructive pul-monary disease (COPD). Reducing the number, frequency and the severity of exacerbations istherefore an important management goal identified by treatment guidelines for both diseases.There are similarities with respect to clinical manifestations, etiology and epidemiology, butthere are also clear differences not the least in how well patients respond to treatment. Thesimilarities and differences between exacerbations asthma and COPD with an emphasis onepidemiology and clinical presentation are discussed and compared in this chapter