  We investigate the generalized second law of thermodynamics (GSL) in
generalized theories of gravity. We examine the total entropy evolution with
time including the horizon entropy, the non-equilibrium entropy production, and
the entropy of all matter, field and energy components. We derive a universal
condition to protect the generalized second law and study its validity in
different gravity theories. In Einstein gravity, (even in the phantom-dominated
universe with a Schwarzschild black hole), Lovelock gravity, and braneworld
gravity, we show that the condition to keep the GSL can always be satisfied. In
$f(R)$ gravity and scalar-tensor gravity, the condition to protect the GSL can
also hold because the gravity is always attractive and the effective Newton
constant should be approximate constant satisfying the experimental bounds.
