Every day, clinicians use imaging to diagnose disease and to assist in therapeutic decision-making. The value of anatomic imaging of lung structure with X-ray computed tomography (CT) is well known to every clinician. Advances in molecular imaging that may eventually be used to guide gene therapy or to evaluate biologic processes in vivo at the cellular and even subcellular levels hold great promise. Recent improvements in instrumentation are making it possible to combine different types of measurements into single multimodality imaging sessions (see Chapter 7).