Attenuation effects are more severe for coincidence imaging than for single-photon imaging. The capability to measure and correct attenuation now exists with dedicated positron emission tomography (PET) scanners. Attenuation correction may or may not improve lesion detection in various situations, but it definitely produces a more realistic radioactivity distribution and is essential for quantitation, which is an important PET capability. For hybrid PET systems, though, which are relatively new and in which neither the performance nor the cost of the scanner can be compromised much compared with the conventional nuclear medicine device alone, attenuation correction is still novel. Just as the entire modality of PET imaging on hybrid gamma cameras has expanded very rapidly, the capability to achieve attenuation correction has quickly followed. Both radioactive source-based and x-ray-based systems exist that provide adequate maps for attenuation correction, and the x-ray systems go even further to provide anatomic detail to aid in image interpretation.