In this issue, we evaluate two instruments that assess the quality of respiration by analyzing blood samples: blood gas/pH analyzers (BGAs) and in vitro multiwavelength oximeters. These complementary technologies, which are being widely used together, give clinicians a more complete, accurate analysis of the patient's overall respiratory condition than is possible with a single device. Below we present a clinical overview of the purpose of both devices and their importance in assessing a patient's well-being. Following the general discussion are the individual evaluations of each device. Many technical terms used in the text and tables are defined in the Glossary and are italicized in text.