  Spin-transfer torque (STT) provides key mechanisms for current-induced
phenomena in ferromagnets. While it is widely accepted that STT involves both
adiabatic and non-adiabatic contributions, their underlying physics and range
of validity are quite controversial. By computing microscopically the response
of conduction electron spins to a time varying and spatially inhomogeneous
magnetic background, we derive the adiabatic and non-adiabatic STT in a unified
fashion. Our result confirms the macroscopic theory [Phys. Rev. Lett.
\textbf{93},~127204 (2004)] with all coefficients matched exactly. Our
derivation also reveals a benchmark on the validity of the result, which is
used to explain three recent measurements of the non-adiabatic STT in quite
different settings.
