More than a catchphrase, molecular imaging is necessary for studying disease progression and the possible effects of drugs on disease at the cellular and molecular levels in living animals. Molecular imaging encompasses techniques in the fields of optical imaging, tomography, fiber-based microscopy, intravital microscopy and radionuclide imaging. Researchers from University Hospital Göttingen in Germany have reviewed molecular imaging techniques for preclinical tumor research.The reviewers state that flat panel volumetric CT is a highly effective imaging modality for tumor research. It can scan an entire animal in 1 mm3, and it enables repetitive and noninvasive imaging. The reviewers call noninvasive imaging of tumor volumes by this technique “a milestone in animal research” because it keeps constant for all animals the experimental variable of treatment onset. In comparison with flat panel volumetric CT, microCT delivers images with higher resolution but requires higher radiation doses, and scans take minutes instead of seconds. Therefore, microCT is used most frequently for imaging parts of specimens rather than entire living organisms. Limitations of microCT and flat panel volumetric CT are that both require the use of contrast agents that are potentially toxic to varying degrees to enhance resolution and picture quality, and that both techniques require histological analysis to classify tumor type.The authors also describe near-IR fluorescence imaging systems and two-photon fluorescence microscopy and conclude that the former permits more accurate measurement of the effects of cancer treatment and progression, providing quantitative measurements of molecular target intensity and location because less autofluorescence and deeper tissue penetration occur at the longer excitation wavelength. Multiphoton microscopy allows deeper in vivo optical imaging in living animals than conventional single-photon optical imaging.The authors also note that fluorescent protein imaging and MRI have been used to monitor tumor growth, vascularization and metastasis. On a separate note, they state that in vivo quantification of tumor cells in living animals remains within reach. (Cytometry A, August 2007, pp. 542-549.)Editor’s Note: In vivo quantification of tumor cells in living animals has been achieved since the submission of this article for publication, in research detailed in “Biophotonics in Medicine” on page 42.