In symbiosis, different organisms have to recognize each other and exchange material and information. While the functional morphology of symbiont cells has extensively been studied little attention has been paid to mutual interactions and cell-to-cell signals. The main topic of the book is the comparison and discussion of the short-distance signals triggering mutualism in animal and plant cells. Thus it provides a) the basis for a better understanding of various aspects of cell compatibility; b) standardisation of a common vocabulary for animal and plant symbiotic biology; c) optimization of experimental techniques for studying signals in symbiosis. The results show the differences and the general principles in the signalling processes of plant, animal and bacterial cells.