This chapter will present the recent progress and challenges of selective area regrowth and doping using metalorganic chemical vapor deposition (MOCVD). The general MOCVD regrowth process will first be described, followed by detailed studies on the regrowth dynamics and doping in trenches with different geometries. A comprehensive material characterization toolkit was developed to investigate the regrown structures and interfaces, including cathodoluminescence (CL), secondary electrons (SEs) in scanning electron microscopy (SEM), electron holography in transmission electron microscopy (TEM), secondary ion mass spectrometry (SIMS), and capacitance-voltage (C-V) measurement. It was revealed that the regrown p-GaN in trenches showed non-uniform acceptor distribution, and the regrowth interface after conventional dry etching exhibited high concentrations of impurities and charges with significant band bending. It was hypothesized that the regrowth interface may behave similarly to a p+-n+ tunneling junction, leading to high leakage in the regrown devices. Two novel etching techniques were studied to mitigate the etching damage at the regrowth interface, including low-power dry etching and atomic layer etching, which show promising results in reducing interface charges and leakage in regrown GaN p-n junctions. Regrown GaN p-n junctions with low leakage and high breakdown voltage and normally-off vertical GaN junction field-effect transistors were successfully demonstrated.