  The nuclear symmetry energy represents a response to the neutron-proton
asymmetry. In this survey we discuss various aspects of symmetry energy in the
framework of nuclear density functional theory, considering both
non-relativistic and relativistic self-consistent mean-field realizations
side-by-side. Key observables pertaining to bulk nucleonic matter and finite
nuclei are reviewed. Constraints on the symmetry energy and correlations
between observables and symmetry-energy parameters, using statistical
covariance analysis, are investigated. Perspectives for future work are
outlined in the context of ongoing experimental efforts.
