Ammonia is investigated as nitrogen precursor for molecular beam epitaxy of group III nitrides. With the particular on-surface cracking approach, NH3 is dissociated directly on the growing surface. By this technique, molecular beam epitaxy becomes a serious competitor to metal organic vapor phase epitaxy. Thermodynamic calculations as well as experimental results reveal insights into the growth mechanisms and its differences to the conventional plasma approach. With this knowledge, homoepitaxially GaN can be grown with record linewidths of 0.5 meV in photoluminescence (4 K). GaN layers on c-plane sapphire also reveal reasonable material properties (photoluminescence linewidth 5 meV, n ≈ 1017 cm−3, μ ≈ 220 cm2/Vs). Beside GaN growth, p- and n-doping of GaN as well as the growth of ternary nitrides are discussed. Using the presented ammonia approach UV-LEDs emitting at 370 nm with linewidths as narrow as 12 nm have been achieved.