Nicotinamide adenine dinucleotide (NAD+) is a coenzyme that is present in all eukaryotic cells. NAD+, as a co-substrate for other kinases such as Sirtuins and PARP, influences the activity of these important kinases and plays an important role in signaling transduction. NAD+ controls a variety of intracellular signaling pathways. In this chapter, we summarize some of the important intracellular signaling reactions NAD+ involved, including protein deacetylation, protein mono- and poly-ADP-ribosylation modifications, the production of cyclic ADP ribose and nicotinic acid adenine dinucleotide phosphate (NAADP), etc. Through these reactions, NAD+ regulates the activities of deacetylase Sirtuin protein family, ARTs, and PARPs and other signaling pathways, which, in turn, regulate a variety of intracellular transduction events, including important intracellular biological processes such as calcium signaling, DNA, RNA, and ribosome modifications.