Advances in molecular beam epitaxy (MBE) have been crucial for the engineering of heterostructures in which the wave nature of electrons dictates carrier transport dynamics. These advances led to the first demonstration of negative differential conductance (NDC) in arsenide-based resonant tunneling diodes (RTDs) in 1974. In contrast to the 17 years elapsed between the initial MBE growth of arsenide semiconductors and the first demonstration of room-temperature GaAs/AlAs RTDs, the development of polar III-nitride RTDs has been remarkably different. After pioneering growths of nitride materials by MBE in 1973, it would take 43 years-until 2016-to demonstrate the first GaN/AlN RTD that exhibits repeatable NDC at room temperature. Here, we discuss, from the crystal growth point of view, the key developments in the epitaxy of III-nitride heterostructures that have led us to the demonstration of robust resonant tunneling transport and reliable NDC in III-nitride semiconductors. We show that in situ tracking of the crystal electron diffraction allows us to deterministically control the number of monolayers incorporated into the tunneling barriers of the active region. Employing this technique, we fabricate various GaN/AlN RTD designs showing the exponential enhancement of the resonant tunneling current as a function of barrier thickness. In addition, we experimentally demonstrate that tunneling transport in nitride RTDs is sensitive to epitaxial parameters such as the substrate growth temperature and threading dislocation density. This new insight into the MBE growth of nitride resonant tunneling devices represents a significant step forward in the engineering of new functionalities within the family of III-nitride semiconductors, allowing to harness quantum interference effects for the new generation of electronic and photonic devices.