  The present thesis is a collection of papers which deals with analytic models
that can drive the present accelerated expansion of the universe, starting from
a decelerated expansion in the early matter dominated regime. Both minimally
and non minimally coupled scalar fields are employed for the purpose.
Interaction between nonminimally coupled scalar field, such as the Brans-Dicke
field, with dark matter and dark energy are considered. This interactions are
shown to solve some fine tuning problems. Curvature driven acceleration is also
considered in this thesis, which shows that a smooth transition from a
deceleration to an acceleration is possible in this scenario.
