Macrocyclic compounds number among them some of the most significant compounds of nature, for example, the porphyrin unit in haemoglobin. The biomimetic formation as well as the artificial synthesis of such macrocycles is often based on simple building blocks like monocyclic pyrrole derivatives. This modular principle is also found in the preparation of phthalocyanines, crown ethers, calixarenes, and several other macrocyclic compounds. The electronic structure, the steric character, and the solubility of these products is important for application, for instance, to ligands of transition metals, pigments, and hosts for guest molecules or ions.