The development of positron emission tomography/computed tomography (PET/CT) scanners has allowed not only straightforward but also synergistic fusion of anatomical and functional information. Combined PET/CT imaging yields an increased sensitivity and specificity beyond that which either of the 2 modalities possesses separately and therefore provides improved diagnostic accuracy. Because attenuation correction in PET is performed with the use of CT images, with CT used in the localization of disease, accurate spatial registration of PET and CT image sets is required. Correcting for the spatial mismatch caused by respiratory motion represents a particular challenge for the requisite registration accuracy as a result of differences in temporal resolution between the 2 modalities. This review provides a brief summary of the materials, methods, and results involved in multiple investigations of the correction for respiratory motion in PET/CT imaging of the thorax, with the goal of improving image quality and quantitation. Although some schemes use respiratory-phase data selection to exclude motion artifacts, others have adopted sophisticated software techniques. The various image artifacts associated with breathing motion are also described.