Nonequilibrium Kondo problem is one of long standing problems in condensed matter physics. For this reason, all of the experimental observations for the tunneling conductance of various Kondo-involved mesoscopic systems are not properly explained. In this colloquium, I will introduce basic Kondo physics in equilibrium and extend the discussion to steady-state nonequilibrium in which entangled Kondo singlets perform coherent tunneling depending on the amount of bias. Then, I clarify that the Kondo phenomenon at steady-state nonequilibrium is the side peaks appearing in dI/dV vs. V curve, and I reproduce the dI/dV line shapes of various mesoscopic Kondo systems such as quantum dot single-electron transistor, quantum point contact, and adsorbed magnetized atom on a metallic substrate. I will show that multilayer graphene and high-Tc superconductor can be treated in the same category. I will introduce the theoretical method to obtain the on-site Green’s function for the two-reservoir Anderson impurity model under bias, which gives the tunneling conductance mentioned above.