  Energy conditions can play an important role in defining the cosmological
evolution. Specifically acceleration/deceleration of cosmic fluid, as well as
the emergence of Big Rip singularities, can be related to the constraints
imposed by the energy conditions. Here we discuss this issue for $f(R)$ gravity
considering also conformal transformations. Cosmological solutions and
equations of state can be classified according to energy conditions. The
qualitative change of some energy conditions for transformations from the
Jordan frame to the Einstein frame is also observed.
