The development of all-solid-state Li-ion batteries (ASSLIBs) has gained significant attention due to their potential for improved safety, energy density, and performance compared to traditional Li-ion batteries. NASICON-type Li-ion conducting solid electrolytes (LICSEs) have emerged as a promising candidate for use in ASSLIBs because of their high Li-ion conductivity and excellent chemical stability. Replacement of the liquid electrolytes from conventional batteries with suitable solid electrolytes not only resolves some of their serious safety challenges, such as leakages and thermal instabilities, but also enhances their cycle life and power densities. One of the critical challenges in realizing all-solid-state batteries is not only the development of thermally and electrochemically stable solid electrolytes exhibiting good ionic conductivities but also reducing the impedance associated with the electrolyte-electrode interfaces in the device. NASICON-type solid electrolytes exhibit better stabilities compared to garnet-based systems, and they also exhibit better conductivity values in comparison to perovskite-based materials. This chapter shows the recent advances and challenges of NASICON-type LICSEs, LiM2(PO4)3 (M: Zr, Ti, and Ge), and their applications in all-solid-state batteries. Various strategies known to achieve solid electrolytes with improved ionic conductivity values by doping with dopants of different oxidation states-and further, the correlation of their transport results with respect to their crystal structure-are reviewed. The use of LiM2(PO4)3-based solid electrolytes in various polymer matrices for the advancement of composite-polymer electrolytes is underlined. Various synthesis methods known for the development of LiM2(PO4)3-based polycrystalline materials and composite-polymer electrolytes are summarized. We also provide an outlook for future research and development of NASICON-type LICSEs for use in ASSLIBs. Overall, this chapter highlights the potential of NASICON-type LICSEs as a key component for the development of high-performance ASSLIBs.