This manuscript constitutes a first attempt to categorize the cell receptors. Based on evolutionary and biological characteristics, it is possible to classify ligand-receptor units into original families. Certain dynamic patterns that could not be classified previously lead us towards the ligand-receptor unit. Biological information is transformed at the receptor by cellular transduction and effector pathways. The dynamic code is compared with the genetic code. The dynamic code controls the biological information patterns necessary for the integral function and structure of cells, tissue, organs, and organisms. The genetic code controls the assembly and molecular structure of proteins. The original families can be assigned certain original functions. A coevolution of ligand-receptor and dynamic code is revealed. For the time being the most useful approach to receptor classification would seem to be based on the organization of the molecular and protein structure of receptors. Two classes of receptor can be described: receptors for cyclic hydrophobic compounds and membrane receptors. Until we know of the families of single membrane-spanning receptors, fourfold membrane-spanning receptors, sevenfold membrane-spanning receptors, fourfold membrane-spanning receptors. For the sevenfold membrane-spanning receptor the identity of conformation and dynamics of biological function has been established. Light and peptide ligands are used as examples. A systems theory for information transduction at receptors is introduced which can also describe the processes of sensitivity modulation.