The lecture looks at how cells in emerging multicellular organisms have evolved ways of communicating with each other. The basic ‘yes’ and ‘no’ signalling was probably mediated by release into the extracellular space of substances which were available in abundance inside the cells – purine nucleotide ATP (molecule charged with energy – excitatory Yang) and its breakdown product adenosine (molecule devoid of energy – inhibitory Yin). The lecture will use examples from current research demonstrating how this dual system of conveying information from one cell to another has been preserved during evolution. Both substances are important modulators of cellular functions still playing often opposing roles in the peripheral tissues as well as in the central nervous system.