Dale's 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.