Although of some value for understanding etiologic mechanisms, the classic diagnostic categories of asthma and emphysema and especially chronic bronchitis have not served clinicians well for defining prognosis and therapeutic options. Until more useful diagnostic categories are available, the choice of diagnostic tests should be guided more by their clinical usefulness than by their sensitivity and specificity for identifying classic diagnostic categories of obstructive lung disease. A history consistent with asthma is as good evidence of asthma as that provided by most tests, especially if combined with spirometric evidence of complete reversibility of episodes of obstruction. Positive bronchial challenge studies and partial responses to bronchodilators are common in asthma but of limited diagnostic specificity. Tests of allergic function are of limited specificity for asthma, although a low IgE level is rare. Findings of reduced expiratory flows, high TLC, and low DLCO, or radiologic signs of hyperinflation, bullae, and pulmonary vascular deficiency pattern are useful for diagnosing cases of severe emphysema, but they are of limited sensitivity for the detection of mild to moderate disease. Advances in high resolution CT offer promise of earlier diagnosis of emphysema. Making a diagnosis of chronic bronchitis based on defined criteria for chronic sputum production is easy but of limited clinical value. Prospective longitudinal studies and advances in technology promise more clinically useful diagnoses in the future.