In the Standard Model (SM), baryon number – lepton number (B-L) is perfectly conserved.  Therefore, the observation of B-L violation reveals the existence of physics beyond the SM. Traditionally, given the severe experimental constraints on ∆B =1 processes, B-L violation with baryons is probed via neutron-antineutron oscillations, although this process suffers from quenching in the presence of external fields or matter. I will discuss another possibility, neutron-antineutron conversion, in which the ∆B=2 process appears with an external source. I will start with the Lorentz invariant B-L violating operators of lowest mass dimension and show how the appearance of constraints on the “arbitrary” phases in the discrete symmetry transformations help restrict the possible low energy neutron-antineutron transformation operators. Then I will discuss the connection between neutron-antineutron oscillations and conversions --- and how neutron-antineutron conversion can provide a complementary probe to neutron-antineutron oscillation experiments. Finally, I will discuss possible neutron−antineutron conversion proposals and explicitly show how neutron-antineutron conversion experiments can set limits on the scale of B − L violation.