A review in which recent developments in ammonium ion complexation in the past decade were summarized, with new theor. and fundamental phys. org. work and further optimizations of existing sensor systems. Emphasis was placed on peptide and protein receptors, which were still in their infancy but will become increasingly important in the near future. An overview was given of artificial receptors developed for the other biol. important types of org. cation, i.e., amidinium, guanidinium and pyridinium cations. Each subdivision included biol. examples of the mol. recognition of the resp. guest species, introduced new synthetic receptor systems, their binding characteristics and selectivity profile, and (potential) applications.