This book focuses on the construction of redox systems composed of doubly fused para-quinodimethane (p-QD) units and on the elucidation of the unique redox behavior. p-QD is a class of non-aromatic π-conjugated compounds that is well-known to be interconvertible scaffolds in many response systems due to its contribution of resonance structure with a planar aromatic sextet ring. This book describes studies on a strategy for precisely controlling the frontier orbital levels of p-QD-based redox systems and a nature of cationic states of π-electron systems with doubly fused p-QD units.