Ammonia (NH₃) is a clean energy source that can either be directly used as fuel or a hydrogen carrier due to its high energy density and high hydrogen content. The NH₃ electro-oxidation reaction (AOR) is the main reaction in both direct NH₃ fuel cells and NH₃ electrolysis. The AOR is thermodynamically favorable; however, the sluggish kinetics of the reaction can result in issues such as high overpotential, slow reaction rate, deactivation, etc. To overcome this, multiple strategies have been discussed to develop electrocatalysts that maintain a robust reaction rate in low overpotential regions. In this review, the fundamentals of AOR, including thermodynamics, kinetics, and experimental techniques, are studied. This review also focused on recent progress for catalyst modifications and their effects, with a particular focus on Pt- or Ni-based electrocatalysts. Additionally, vacant rooms needed to be developed was pointed, and a way to overcome the limitations was suggested. The fundamentals and efforts to prepare catalysts reviewed in this work will be effective in proposing and designing new robust electrocatalysts leading to advance AOR in practice.