  We present a derivation of Hawking radiation through tunneling mechanism for
a general class of asymptotically flat, spherically symmetric spacetimes. The
tunneling rate $\Gamma \sim \exp{(\Delta S)}$ arises as a consequence of the
first law of thermodynamics, TdS=dE + PdV. Therefore, this approach
demonstrates how tunneling is intimately connected with the first law of
thermodynamics through the principle of conservation of energy. The analysis is
also generally applicable to any reasonable theory of gravity so long as the
first law of thermodynamics for horizons holds in the form, TdS=dE + PdV.
