Host-compounds recognize and incorporate specific molecules, atoms or ions (guests) into the molecules to form complexes. In the formation of these complexes, a variety of forces such as electrostatic interaction, hydrophobic interaction, hydrogen bonding, etc. are utilized to create high selectivity. Molecular sensors, synthetic enzymes, separation systems, etc. utilize the high guest selectivity of these host compounds. Synthesis of new host compounds with even higher selectivity are currently being developed. For example, Shinkai and co-workers have developed exciting purification techniques for fullerene C60 employing calixarene. According to these techniques, the stirring of crude fullerene and tert-butylcalixarene in toluene causes only C60 to be incorporated in the calixarene to form and precipitate a complex. Filtration and subsequent stirring of the complex in chloroform cause it to decompose to yield C60 of high purity as a precipitate. This method has been reported to be an eﬃcient puriﬁcation technique in obtaining C60 compared with conventional column puriﬁcation techniques.