Information is transmitted throughout the nervous system by chemical and electrical signals. The cacophony of neuronal signals does not end at the cell surface, but continues deep into cells via diffusible intracellular messenger molecules. The list of molecules that serve as intracellular messengers continues to expand, but several stand out as very common examples, including Ca2+, cyclic nucleotides (cAMP and cGMP), and inositol phospholipids and their metabolites (IP3, diacylglycerol, etc.). Cellular responses to extracellular neurotransmitters are often mediated by one or more of these intracellular messengers, affording several functional advantages. Intracellular messengers allow spatial spread of signals within cells, contribute to amplification of signals, and allow responses to persist even after the neurotransmitter is gone from outside the cell. Intracellular messenger systems are also advantageous because they participate in a great degree of “crosstalk,” increasing the repertoire of responses to a given external stimulus and allowing for biochemical computation at the level of an individual cell.