Advances in genomics, proteomics and technology are changing medicine in fundamental ways. There are increasing clinical and laboratory requirements to obtain cellular and molecular information in vivo. This is particularly true in oncology, where the behavior of tumor cells is inextricably linked to their milieu. If cancer cells are removed from their microenvironment, their pattern of gene expression changes. Therefore, non-invasive, quantitative means of detecting gene and protein activity are essential. In vivo imaging is one methodology for achieving this. Marked advances in tracer methods for PET scanning or single-photon nuclear medicine techniques have occurred in the past few years. MRI contrast agents that reflect physiologic information are also being developed, although larger mass quantities of injectable material are required. The useful concept of ?activatable agents? was pioneered in MRI. Similarly, ultrasound and computed tomography are being re-engineered to reflect information at the cellular level. In vivo optical imaging technologies have matured to the point where they are indispensable laboratory tools for small animal imaging. Human applications are in the feasibility testing stage, and the future for clinical optical imaging techniques looks bright. Merging these molecular imaging techniques with minimally or non-invasive image-guided therapeutic delivery techniques is a subsequent goal in the fight against cancer.