Accurate live tumor imaging in mice is now possible by means of high-resolution positron emission tomography (micro-PET) and X-ray computed tomography (micro-CT). By providing a powerful tool to examine biological samples with complex structure in vivo, this technology generated a significant advance in the cancer research field, particularly regarding the ability to perform longitudinal studies in combination with a therapeutic intervention. Here, we describe methods to optimize visualization of murine lung tumors by micro-PET, micro-CT and combined micro-PET-CT.