Heterocycles or heterocyclic aromatic compounds are a class of aromatic compounds in which heteroatoms replace one or more of the ring carbons. This chapter focuses on nitrogen, oxygen, and sulfur heterocycles. It also discusses reduced heterocycles, which are cyclic molecules that contain nitrogen, oxygen, or sulfur. Alkaloids are a class of nitrogenous organic compounds of plant origin that have pronounced physiological actions on humans. Imidazole and pyrazole are important units in many pharmaceutical preparations and also in naturally occurring compounds. Pyrazine, pyrimidine, and pyridazine derivatives are common components of pharmaceutically important compounds. Cyanuric chloride is the main precursor to the herbicide atrazine, a widely used to prevent pre-emergence broadleaf weeds.