Dales Principle postulated that a neuron functions as a metabolic unit, whereby a process occurring in the cell can influence all of the compartments of that given neuron. This was unfortunately transformed in the literature to a principle stating that “a single cell releases only one neurotransmitter�. Until recently, this has influenced many neuroscientists to consider with skepticism the idea that classical neurotransmitters could be co-released from neurons. It is now clear that the “one neuron, one neurotransmitter� postulate is the exception rather than the rule. The aim of this book is to gather the available evidence, provided by the authors that have discovered and studied the co-existence or co-release of several pairs of neurotransmitters, in several neural networks. It will provide a timely overview of a mechanism of neural communication that is likely to attract increasing attention of the neuroscientific community.