Ultrafast laser pulses heat electrons to thousands of Kelvin in femtoseconds while the lattice stays cool — electrons and phonons are far from equilibrium. The two-temperature model (TTM) couples them: C_e dT_e/dt = −G(T_e−T_l)+S(t) and C_l dT_l/dt = G(T_e−T_l), where G is the electron-phonon coupling constant.