The prototypical second messenger cAMP has been implicated in a wide variety of (often contradictory) physiological processes, including proliferation, apoptosis, development, differentiation, ion transport, pH regulation, migration, and gene expression. We now know cAMP signaling is compartmentalized, and cells are comprised of multiple, independently-regulated cAMP signaling microdomains. Because each cAMP microdomain mediates a distinct cellular response, compartmentalization allows this single second messenger to simultaneously mediate multiple (often disparate) processes within an individual cell. Cyclic AMP is produced from ATP by adenylyl cyclases (ACs), and different microdomains are defined by their mechanisms of second messenger generation. There are two distinct families of ACs in mammals; G protein regulated transmembrane adenylyl cyclases (tmAC) and soluble adenylyl cyclase (sAC). TmACs anchor cAMP signaling microdomains at the plasma membrane, where they respond to hormonal signals operating via G protein coupled receptors. In contrast, sAC can be found in the cytoplasm and inside cellular organelles. Due to its unique localization, sAC defines multiple, independently regulated intracellular cAMP signaling microdomains, including in the nucleus and inside the mitochondrial matrix. In contrast to the family of plasma membrane-bound, hormone-regulated tmACs, sAC activity is directly regulated by bicarbonate (HCO3-) and calcium (Ca2+) ions, and its activity is sensitive to physiological fluctuations in cellular ATP. Due to the ubiquitous presence of carbonic anhydrases (CA), which catalyze the instantaneous equilibration of carbon dioxide (CO2), HCO3-, and protons, mammalian sAC, and its HCO3--regulated orthologs throughout the kingdoms of life, serve as Nature’s physiological CO2/HCO3-/pHi sensors. Mammalian sAC is widely expressed. Among other functions, sAC has been demonstrated to be responsible for the cAMP-dependent signaling cascades which regulate sperm activation; insulin secretion from pancreatic cells; ciliary beat frequency in airway epithelia; luminal pH in the epididymis and kidney; the mitochondrial electron transport chain; activity dependent feeding of neurons; axonal outgrowth; acidification of lysosomes, and regulation of intraocular pressure.