The development and investigation of various alternative fuels are essential due to the uncertain future of fossil fuels and the significant exhaust emissions from their combustion in internal combustion engines. The issue of global warming, due to the emission of greenhouse gases from fossil fuel combustion, is also a serious concern for the planet. Ammonia and hydrogen are examples of carbon-zero alternative fuels that reduce the amount of gaseous and particulate emissions from internal combustion engines. These fuels have been tested in their pure forms as well as blended with fossil fuels. Ammonia (NH3) is an attractive carbon-free alternative fuel that reduces CO2, SOX, soot, and hydrocarbon pollutants. Its high-octane number permits the advancing of spark timing which offers an excellent knock resistance property. Also, the ease of production, storage, and transportation makes it a viable option. The complete combustion of ammonia produces only nitrogen and water, but due to oxidation of N2 at elevated temperatures, it produces thermal NOX. The challenges such as high N2O, unburned ammonia, and hydrogen cyanide (HCN) emissions due to fuel-bound nitrogen must be addressed for effective ammonia combustion in internal combustion engines. Studies indicate that a blending ratio of 40% ammonia and 60% diesel in a dual-fuel method lowers NOX emissions compared to pure diesel combustion, also advancing fuel injection timing and multiple fuel injections reduces the N2O and NH3 emissions. The blending of 50% ammonia and 50% CNG reduces the CO2 emissions by 28%. Hydrogen cyanide emissions can be mitigated by advancing the combustion phase closer to TDC and heating the intake air. Further, with the application of after-treatment devices the emission of NOX and unburned NH3 can be minimized. This chapter provides a detailed review of the emission characteristics of ammonia-powered engines.