A zonal isolation device comprises a packer assembly and an internal setting mechanism operable to actuate the packer assembly from an unset position to a set position, wherein the zonal isolation device is resettable and retrievable. Another zonal isolation device comprises a packer assembly, a setting mechanism operable to actuate the packer assembly from an unset position to a set position in response to hydraulic pressure alone, and a locking mechanism operable to lock and unlock the packer assembly from the set position in response to hydraulic pressure alone. A method for setting a zonal isolation device within a well bore comprises running the zonal isolation device in an unset position within the well bore on a work string, applying a differential pressure between the work string and the well bore, and actuating the zonal isolation device to a set position in response to the differential pressure alone.