The [2 + 2] cycloaddition-retroelectrocyclization (CA-RE) reaction, which generally occurs between electron-rich alkynes and electron-deficient alkenes, has been extensively investigated because of the characteristics of click chemistry, such as high yield, no catalysts, no side reactions, and mild conditions. Therefore, it has been employed as a powerful tool for the construction of donor-acceptor (push-pull) chromophores with tunable energy levels and absorption properties, as well as for the postfunctionalization of polymers. In this chapter, the development and mechanism of the [2 + 2] CA-RE reaction are summarized, and its application to the synthesis of molecular and polymeric push-pull chromophores is reviewed. In addition, functional applications of CA-RE products such as nonlinear optical materials, photovoltaic materials, p-dopants for organic semiconductors, and colorimetric sensors are briefly presented.