  In a homogenous and isotropic cosmology, we introduce general exact solutions
for some modified gravity models. In particular, we introduce exact solutions
for power-law $f(R)$ gravity and Brans-Dicke theory in Einstein and Jordan
conformal frames. In the Brans-Dicke case, the solutions are presented for both
single and double exponential potentials in Einstein frame which correspond to
power-law potentials in Jordan frame. Our analysis for extracting general exact
solutions can also be generalized to those scalar-tensor theories in which the
scalar field has an exponential coupling to Ricci scalar.
