In the following section, we shall discuss the conceptual strength of combining CARS spectroscopy with laser scanning microscope technology. As already mentioned the resultant technique called CARS-microscopy combines the potential of Raman scattering in recording chemical images of an unstained sample (i.e., the image contrast is based on the vibrational signature of the sample itself) with the benefits of coherent anti-Stokes spectroscopy (i.e., a coherently amplified and directed signal with no disturbance from autofluorescence of the sample). In the preceding section it has been explained that the CARS signal is directed and the direction is determined by the phase-matching condition (). This phase matching condition is trivially fulfilled in gaseous media, in which dispersion does not play a significant role, so that collinear excitation geometry can be used. In contrast the application of CARS spectroscopy in dispersive media, e.g., liquids, requires in general the application of noncollinear phase matching geometries.9 However, the application of these noncollinear geometries in microscopy is hindered by their complexity.18–20 Thus, the combination of CARS spectroscopy as a coherent nonlinear optical process, during which phase-matching needs to be fulfilled, with laser-scanning microscopy raises the question, how phase matching can be met in an optically dense medium, e.g., a tissue section. Here, Zumbusch and colleagues21 showed in their seminal work that CARS microscopy is possible in a collinear optical geometry, i.e., the direction of the -vectors of pump-, Stokes- and signal-beam coincide, if a microscope objective with a large numerical aperture is used (typically ). Here, the large angle of incidence causes the presence of a Stokes-beam -vector, , to fulfill phase matching for every vector present in the focus. Furthermore, if this tight-focusing regime is met in CARS microscopy, the CARS signal itself is generated only over a very short interaction length () of the incident lasers with the sample. In other words in such a tight-focusing regime, the CARS signal stems from a very constrained spatial volume. The CARS signal can be detected either in the forward (F-CARS) or in backward (EPI-CARS) direction21–23 Since the interaction length in the sample is short, all scattering objects lead to large F-CARS signals as the phase mismatch () is small [see also Eq. (9)]. This situation is different for EPI-CARS, where for large objects the phase-mismatch is significant. Thus, EPI-CARS signal is only generated from small objects, that is, where is small enough to develop the function within the expression for the CARS intensity [see Eq. (9)] to the maximum. Though at the first glance this seems to present a severe disadvantage, EPI-CARS can be used experimentally to exclusively monitor small objects embedded in a matrix as introduced by Xie and coworkers.22 However, it has to be mentioned that for turbid samples, the F-CARS signal might be backscattered to generate an EPI signal.