With the realization that the Maxwell equations are time-reversible,23 the concept becomes quite simple. The wave front emitted by a coherent point source and coupled into a waveguide (the input wave front) emerges from the other end as a wave front (the output wave front) that bears no resemblance to the input wave front [Fig. 1(a)]. Nevertheless, if one lets a time-reversed copy of the output wave front propagate back through the same waveguide, it will resemble a time-reversed copy of the input wave front, in other words, it will focus on the original point source [Fig. 1(b)]. By extension, this means that one can associate each distal point with a proximal wave front, which, after propagation through the waveguide, will focus on that distal point [Fig. 1(c)]. This can be formalized with the transmission matrix formulation.24–26 This means that spatial information—perhaps counter-intuitively—is not lost during propagation through the waveguide and that the spatial information (in other words, the image of the object) can be recovered, e.g., by point-scanning imaging, which is sketched in Fig. 1(d), but other wave front shaping methods, computational methods, or a combination of the two can also be used.27,28 The above rationale holds for waveguides with many degrees of freedom, up to the spatial and Fourier filtering effects associated with coupling into the waveguide. We might also add that the above concept really is a generalization of the imaging setup; in the setup, the first lens performs a physical Fourier transform of the object, and the second lens performs the inverse Fourier transform, which reconstitutes the image of the object; in the generalization, the waveguide performs some complex transformation and the wave front shaping/computation stage performs the inverse of the same transformation, again, reconstituting the image. With this understanding of the concept, the visions of the so-called “lensless endoscopes” appear quite readily: (i) as a waveguide can transmit an image without any additional elements attached to the distal tip, there is nothing that increases the bulkiness of this fiber probe beyond the diameter of the waveguide itself; with a standard optical fiber of outer diameter, this would allow for ultrathin imaging probes, potentially capable of noninvasive deep-tissue endoscopic imaging; (ii) standard optical fibers are highly flexible, and the lensless endoscope concept in principle works no matter the geometrical configuration of the waveguide, this flexibility could evidently be an advantage in many applications; (iii) in the lensless endoscope concept, one has complete control over the output wave front (by intermediary of the input wave front), which might enable enhancement of endoscopic deep-tissue imaging with the methods currently being developed for adaptive optics-assisted microscopy;10,29 (iv) another consequence of the control over the output wave front might be photostimulation with shaped light in an endoscopic setting; and (v) finally, the waveguides with many degrees of freedom used in lensless endoscopes generally have very low nonlinearity; hence it can be expected that they will support high-intensity ultrashort pulses without distorting them through nonlinear effects. This could enable many kinds of nonlinear imaging modalities in endoscopes, which rely on ultrashort excitation pulses.