We give a complete and rigorous proof of the Unruh effect, in the following
form. We show that the state of a two-level system, uniformly accelerated with
proper acceleration $a$, and coupled to a scalar bose field initially in the
Minkowski vacuum state will converge, asymptotically in the detector's proper
time, to the Gibbs state at inverse temperature $\beta=\frac{2\pi}{a}$. The
result also holds if the field and detector are initially in an excited state.
We treat the problem as one of return to equilibrium, exploiting in particular
that the Minkowski vacuum is a KMS state with respect to Lorentz boosts. We
then use the recently developed spectral techniques to prove the stated result.