Nonlinear frequency conversion techniques, which are one of the earliest studied nonlinear optical phenomena, have been developing for several decades. The principles and applications of nonlinear frequency conversion have become increasingly mature with time. In this study, new phase-matching principles associated with nonlinear frequency conversion are proposed and demonstrated. In addition, nonlinear frequency conversion has been reinvigorated by the continuous development of integrated photonics, structural photonics, and quantum optics and has played an irreplaceable role in various fields. Further, we focus on nonlinear frequency conversion and introduce its new principle, platform, and application. Based on the research of our group, we introduce the recent progress of nonlinear frequency conversion in related fields. The contents of this study include the new principle of phase matching at the nonlinear interface, nonlinear harmonic manipulation using structured light, integrated nonlinear optical new platform of lithium niobate thin film, and various new applications such as single-photon frequency conversion and optical quantum interface.