Image registration is a correlation procedure that allows the matching of images obtained by different imaging modalities, both structural and functional. Once registered, they can be combined in a single "fused" image. This technique can be used to improve the interpretation and the quantification of relatively low resolution images obtained in nuclear medicine or to achieve a better diagnosis and treatment planning when high resolution anatomical images are registered. Matching images obtained by different imaging modalities is a major challenge and the various registration techniques are briefly described. The extracranial head and neck region is a challenging field for image registration. A preliminary retrospective experience is presented emphasizing the technique feasibility in a routine clinical context. 2-D and 3-D approaches were used. The requirements for transferring registration technology from the image processing laboratory to clinical practice are discussed, and the contribution of this method to medical interdisciplinary communication is emphasized.