**5. Conclusions**

In this paper, we present an overview of the latest advances in the area of nonlinear frequency generation in WGMR. The review is limited to microspherical solid and hollow WGMR and Kerr phenomena. The Kerr effects we reviewed are third-harmonic generation, third-order sum-frequency generation, frequency combs, Kerr switching and Two-Photon Fluorescence. Stimulated Raman Scattering and Stimulated Brillouin Scattering and combination of other nonlinear phenomena such as FWM are discussed in separated subsections. WGMR are excellent platforms to understand how light, sound and matter interact. The ultimate goal of the nonlinear research in photonic devices is few or single-photon interactions to pave the way for quantum compact photonic devices at room temperature.

**Author Contributions:** Conceptualization, S.S.; writing—original draft preparation, S.S.; writing—review and editing, G.F., D.F., G.N.C.; figure preparation, D.F. Figures 7 and 8 are original unpublished data from our labs, the picture and data were taken by D.F., the experiment was conceived by G.N.C. and the data analyzed by S.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was partially funded by Centro Fermi project MiFo.

**Acknowledgments:** Franco Cosi is gratefully acknowledged for his technical support.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
