We devised a multimodal planar imaging system for in vivo mouse imaging, employing four modalities: optical imaging, green and red fluorescence reflectance imaging, radionuclide imaging and X-ray radiography. We are testing separately, and then in a combined way, each detection mode, via in vivo mouse imaging, with the final purpose of identifying small implanted tumor masses, of providing early tumor detection and following metastatic dissemination. We describe the multimodal system and summarize its main performance, as assessed in our research work in the various stages of the development, in fluorescence and radionuclide tests on healthy or tumor bearing mice. For gamma-ray detection we used a semiconductor pixel detector (Medipix1 or Medipix2) that works in single photon counting. Laser-induced fluorescence reflectance imaging was performed in vivo using a pulsed light source to excite the fluorescence emission of injected hematoporphyrin (HP) compound, a CCD camera, a low pass filter and a commercial image analysis system. The bimodal system was used for the acquisition of combined images of the tumor area (fluorescence: animal top view; radionuclide: bottom view). It was shown that the tumor area can be imaged in a few minutes, with a few millimeter resolution (1 mm pinhole diameter), radioactively (99mTc radiotracer), and with the fluorescence system and that, in one case, only one of the two modalities is able to recognize the tumor. A phantom study for thyroid imaging with 125I source embedded in a simulated tissue indicated a spatial resolution of 1.25 mm FWHM with a 1 mm pinhole.