In addition to genetic determination of mating types, sexual differentiation and development depend on the exchange of communication molecules between complementary cells, hyphae, or structures. In the phylogenetically basic organismic groups, dealt with in this chapter, soluble as well as volatile molecules are involved in partner recognition and in the onset of sexual differentiation. These primary recognition systems contribute substantially to distinction between identical and foreign species and, in addition, must reliably provide recognition of complementary mating types. These signals need to be synthesised and sent out at appropriate stages of development and ideally allow distinction from potentially interfering environmental signals. They need to be perceived by the partner, interpreted, and finally used for initiating adequate developmental programmes. Apart from these basic attributes of communication via pheromones, there is essentially little similarity between the molecules involved in different fungal groups, and even within these groups, different communication signals are often revealed.