In this talk, I will present the various singular limits of the modulated
nonlinear Klein-Gordon equation (the so-called relativistic nonlinear wave
equation). For the semiclassical limit, $\hbar\to 0$, the limit wave
function is described by the
relativistic wave map and the associated phase function satisfies a linear
relativistic wave equation, and the nonrelativistic limit,
$c\to \infty$, is the defocusing nonlinear Schr\"odinger equation. The
nonrelativistic-semiclassical limit, $\hbar\to 0,
c=\hbar^{-\alpha} \to \infty$ for some $\alpha>0$, is the classical wave
map for the limit wave function and typical linear wave
equation for the associated phase function.