This chapter presents a review that synthesizes the published literature on the basic chemistry of betalains and their sources, chemical stability, and functional properties. It has been proven that different sources of betalains such as prickly pear, beetroot, Amaranthus tricolor, and red pitaya have properties related to anticarcinogenic, antioxidative, hypoglycemic, and immunomodulatory effects, all of which could be directly relevant for health benefits. Betanin has shown antioxidant activity in biological lipid environments in vitro, for example, in human low-density lipoproteins and cell membranes. Chemical structures of betalains are based on the immonium conjugates of betalamic acid with cyclo-DOPA and amino acids or amines. The betalamic acid is the chromophore of all the betalain pigments, and the nature of the residue (cyclo-DOPA or amino-acid/amine) that is condensed to betalamic acid determines the type of betalain, i.e. betacyanin or betaxanthin. The chapter shows a general schematic pathway of betalains biosynthesis.