In the past decade, the integration of anatomic imaging and functional imaging has emerged as a new and promising diagnostic tool. Developments in software provided methods to integrate various modalities, such as PET, CT, MRI, and MR spectroscopy. The introduction of combined PET/CT scanners has boosted image fusion in this specific field and raised high expectations. Image fusion can be performed at 3 different levels: visual fusion, software fusion, and hardware fusion, each having strengths, weaknesses, and issues inherent to technique. Visual fusion is the traditional side-by-side reviewing of 2 separate modalities. Software image fusion provides evaluation of 2 modalities in 1 integrated image set. True hardware fusion of PET and CT does not exist at present. Currently, hardware fusion refers to a PET/CT scanner that consists of separate scanners, which positioned in line at a fixed distance, with projection of the PET image over the CT image. The suggested superiority of hardware fusion with these so-called hybrid PET/CT scanners over software fusion has sparked debate. Because scientific data that unequivocally show that state-of-the-art software fusion is less accurate than hardware fusion (as provided in hybrid PET/CT scanners) are unavailable, the primacy of a combined PET/CT scanner over stand-alone PET and CT is more a matter of belief than of science. Further research comparing the overall performance of PET/CT scanners with that of separate scanners with software for image fusion is much needed. The continuous development of better software for image fusion and respiratory and cardiac gating is also needed, not only for PET and CT imaging but also for fusion of PET with MRI and CT with MRI.