Nitrogen (N) is one of the key nutrients for algal growth and is an integral part of many cellular components, for example, in proteins. Being able to determine the inorganic and organic pools of N is consequently critical for algal cultivation. In this chapter, we present the methods we use for determining dissolved inorganic nitrogen (DIN), dissolved organic nitrogen (DON), and particulate organic nitrogen (PON). The methods presented here for DIN rely on colorimetric methods, and those for DON and PON on filtration and high-temperature catalytic oxidation.