NAD⁺ plays central roles in energy metabolism as redox carrier. Recent research has identified important signalling functions of NAD⁺ that involve its consumption. Although NAD⁺ is synthesized mainly in the cytosol, nucleus and mitochondria, it has been detected also in vesicular and extracellular compartments. Three protein families that consume NAD⁺ in signalling reactions have been characterized on a molecular level: ADP-ribosyltransferases (ARTs), Sirtuins (SIRTs), and NAD⁺ glycohydrolases (NADases). Members of these families serve important regulatory functions in various cellular compartments, e.g., by linking the cellular energy state to gene expression in the nucleus, by regulating nitrogen metabolism in mitochondria, and by sensing tissue damage in the extracellular compartment. Distinct NAD⁺ pools may be crucial for these processes. Here, we review the current knowledge about the compartmentation and biochemistry of NAD⁺-converting enzymes that control NAD⁺ signalling.