During the in vivo imaging sessions, the image plane of the MPME was positioned to 30 μm below the tissue surface to obtain the en face unstained, unaveraged in vivo MPME images shown in Figs. 3 and 4. Due to axial motion of the live tissue relative to the MPME image plane, images acquired may be above or below this depth. The recognizable tissue features were highly consistent in all imaged rats. These images show many features that are recognizable in histological tissue samples.22Figure 3(a) shows an image of intrinsic fluorescent signal in the rat kidney. In this image, optical cross-sections of tubular nephrons are visible in the periphery of the organ along with features such as cells, renal tubules, renal interstitium, and renal lumen. Figure 3(b) shows an image of intrinsic fluorescent signal from the interior wall of the rat colon. This image shows an optical cross-section of a colon crypt. Figure 4(a), 4(b), and 1 show intrinsic fluorescence (pseudocolored green) and SHG (gray) images to 30 μm below the surface of the rat liver. These images show hepatocytes (i.e., functional liver cells) surrounded by a collagenous tissue capsule. The hepatocytes are arranged in cords, forming structural units. The blood-filled spaces between the cords are sinusoids. Since blood is a strong light absorber, we see an absence of intrinsic signal in the sinusoid. Strong SHG signal can be seen in the septa of the liver. Images in Figs. 3 and 4 were interpreted with the assistance of a certified pathologist. When imaging kidney, liver, and colon tissue with this device, over of recorded images were free of streaking or warping of features within the image frame even though the organ moves relative to the MPME due to respiration and heart beat, as shown in 1. This can be credited to rigid mounting of the endoscope during image acquisition, isolating tissue while imaging, the image acquisition speed, and the high uniformity of the resonant-nonresonant fiber scanner. The demonstrated device can be further improved by achieving faster frame rates with high signal-to-noise ratio, distal axial sectioning, larger image FOVs while maintaining high-image resolution, and decreasing the device size. Several recent developments are designed to address these issues. For example, by incorporating lensed fibers a larger FOV can be achieved in a miniature endoscope.23 Furthermore, a higher frame rate and axial sectioning can be achieved by incorporating a multifocal approach in the MPME.24 To the best of our knowledge, this research demonstrates the first multiphoton images from unstained tissue in a live animal using a compact and flexible MPME device. These images show many of the features that are commonly seen in biopsied histopathology slides from these tissues, indicating the potential of the MPME device for in vivo diagnostics of tissue health.