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Heterophenes have contributed a great deal to the progress of Π-conjugated systems such as benzenoid and quinoid compounds. Introduction of heterophenes in place of benzenoid or benzoquinoid structures in Π-conjugated systems often leads to improvement of properties and stability as well as facile synthesis. In recent studies on phosphorus compounds, heterophenes such as thiophene and selenophene played a key role in the construction of unique Π-conjugated systems. The focus of this chapter is on heterophenes carrying phosphorus functional groups as key structures. A series of phosphaquinoid compounds, which have low coordinated phosphorus as well as quinoid structure, have been synthesized during this decade, and those carrying thienoquinoid structure greatly contributed to diversity as well as understanding of properties. Cyclic Π-conjugated systems completely substituted by heteroatoms have attracted considerable attention for a long time from structural as well as physical viewpoints. Although benzene carrying phosphorus substituents on all carbons has not yet been synthesized, recent studies on all-phosphorus-substituted aromatic compounds reached five-membered ring systems, and thiophene and selenophene fully substituted by phosphoryl groups were synthesized by unconventional manner and shown to have unique structure. [ABSTRACT FROM AUTHOR]