Book excerpt: Inspired by naturally occurring light-active molecular systems such as photosynthesis, scientists have long devoted their efforts to understanding how light and molecules interact. Based on a raft of knowledge on light absorption, energy migration and electron transfer, fluorescence and phosphorescence, and various photochemical reactions, light can now be utilized for energy conversion, information storage, medical applications, and development of next-generation photofunctional materials that cannot be obtained via conventional organic synthesis. This book overviews some of the cutting-edge p-conjugated molecular and polymer materials for organic photovoltaics, artificial photosynthesis, and organic light-emitting devices. It gives insights into the interactions between light and molecules and discusses sophisticated molecular designs, self-assembly and self-organization strategies, and state-of-the-art unconventional analytical methods.