Evaluation and diagnosis of obstructive lung disease call for careful history taking to distinguish asthma from chronic obstructive pulmonary disease (COPD). A history of atopy and intermittent, reactive symptoms points toward a diagnosis of asthma, whereas smoking and advanced age are more indicative of COPD. Clinical features such as cough and wheezing, especially in light of patient history, can be suggestive but are not definitive. Spirometry, usually performed in the primary care setting, is the key diagnostic tool that distinguishes patients with asthma from patients with COPD. In this article, the authors outline an approach to differential diagnosis of asthma and COPD that should result in better evaluation, therapy, and quality of life for these patients.