Cyclic orbital interactions are contained in non-cyclic conjugation as well as cyclic conjugation. For effective interactions, the orbitals are required to meet simultaneously the phase continuity conditions: (1) out of phase relation between electron-donating orbitals; (2) in phase relation between electron-accepting orbitals and between electron-donating and -accepting orbitals. The orbital phase theory is applicable to diverse chemical phenomena of non-cyclic conjugate systems, e.g., relative stabilities of non-cyclic isomers, and selectivities of the reactions through non-cyclic transition structures. The orbital phase theory also includes the rules for cyclic systems, i.e., the Wooward–Hoffmann rule for stereoselection of organic reactions and the Hueckel 4n + 2π electron rule for aromatic molecules. Derivation and applications of the orbital phase theory are reviewed.