Recent progresses on metallic nano-resonators and their nonlinearities are summarized. A three-dimensionally (3D) tapered metallic nano-gap resonator with modal volume of ~1.0 × 10^-7 λ³ is to be discussed in more detail. From a 4-nm-air-gap gold resonator, second harmonic signals are generated in a small space where the electromagnetic energy is highly concentrated. The nonlinear signal is found to be stronger than that from a 100-nm-gap counterpart by a factor of >27,000. Rich nonlinear optical characteristics observed from nm-scale proteins and ZnSe/ZnS quantum dots and that are trapped in the nano-gap will also be discussed. The possibility of photon squeezing into a point-like space could open up a new platform for bio-nanoscopy and small space nonlinear optics with minimal pump power.