<p>Experimental and theoretical data on the [2+2]-photocycloaddition (PCA) and reverse photoinduced cyclobutane ring opening (retro-PCA) reactions are considered. It is shown that for substituted ethylenes having an unsaturated substituent in conjugation with the ethylene π-bond, the mechanisms of forward and reverse reactions are fundamentally different. The forward PCA reaction proceeds in a concerted manner according to Woodward—Hoffman rules in the ππ*-excited state upon excitation of the whole conjugated π-system of the ethylene derivative including the ethylene group. The reverse opening of cyclobutane containing a substituent with a π-system proceeds stepwise by a predissociation mechanism if the substituent π-system rather than the cyclobutane σ-system is excited upon light irradiation. The reaction is initiated by energy transfer from the substituent to the cyclobutane ring, resulting in the formation of a tetramethylene biradical intermediate, which is further either fully cleaved or converted back to cyclobutane. All steps of the forward and reverse PCA reactions and the use of PCA as a mechanism of action of new-type photochromes are discussed in detail in relation to the biphotochromic covalently bonded dyads.</p>