In vivo medical imaging has become one of the most important measurement tools in biomedical research and clinical investigations. Medical imaging applications cover a broad spectrum going from small animal research to human studies. This is due to the fact that in vivo medical imaging techniques based on various modalities are capable of providing anatomic, functional, and metabolic information non-invasively with accurate spatial localization enabling longitudinal studies on the same subject. One of the most significant application of such techniques has been in the study of cancer. In this special issue on in vivo medical imaging, we tried to give the readers a flavor of the types of studies that are being done in cancer research and clinical applications. We should emphasize that the topics covered here due to limited space will only provide the readers with a rather limited view. Furthermore, one of the new and upcoming imaging techniques, namely molecular imaging will not be covered here since there have been various extensive reviews of the field have been given in the literature.