The assessment of pulmonary function, from a clinical perspective, can be performed accurately only if the clinician understands the concept ventilation/ perfusion ratios of the lung. The major categories of ventilation/perfusion ratios are intrapulmonary shunting and physiologic deadspace. Virtually all pulmonary assessments and interventions are aimed to address Qs/Qt or Vd/Vt. This chapter provides background information useful to the clinician in the assessment of intrapulmonary shunting and deadspace analysis. From this information, more thorough assessments of pulmonary function are possible