In mobile SCR applications, the most widespread reducing agent is ammonia. However, due to its toxicity it is not stored directly as pressurized or liquefied gas. Instead, an aqueous solution of 32.5 wt % urea is commonly used as ammonia precursor. The urea solution can be dosed into the main exhaust pipe, where it decomposes in the hot exhaust gas and on the SCR catalyst to yield ammonia and carbon dioxide. Urea as ammonia precursor and its decomposition will be discussed in the first part of this chapter, with a focus on catalytic decomposition and byproduct formation. In the second part, alternative ammonia precursor compounds, including solid ammonia precursors will be presented. These exhibit a higher ammonia storage density, a lower melting point and/or a higher stability when stored at elevated temperatures compared to urea solution.