Asthma severity and asthma control are distinct yet related concepts. Asthma severity describes the underlying disease in the absence of therapy and is ideally defined without concurrent treatment confounding its assessment. Asthma control describes the clinical status of disease in the face of intervention. However, the individual parameters by which we define asthma severity and asthma control overlap significantly. A MEDLINE search between July 1, 2002, and June 30, 2003, was performed using keywords related to concepts of asthma severity of illness and asthma control. From these articles the author selected the articles most relevant for discussion. Asthma is a complex syndrome. Although correlations exist between the various parameters used in clinical assessment, no single parameter can accurately classify all individuals. Assessment of multiple parameters including physiologic measures, symptoms, and activity limitation are necessary to categorize asthma clinical status accurately. In addition, the role biomarkers play in the assessment of disease status is an area of increasing interest. Several validated multidimensional measures for assessing asthma control are now available. Each of these measures includes the parameters of symptoms, activity limitation, and rescue medication use, yet they vary on inclusion of other important components such as physiologic measures and biomarkers. Asthma is a complex syndrome. Currently available instruments demonstrate an improved understanding of the multidimensional approach required to assess asthma control accurately. However, debate continues on the optimal parameters to be included and the role biomarkers play in the clinical assessment of asthma.