Photochemical and photophysical phenomena in molecular systems occur around us at every moment. Light absorption initiates the chemical reactions at the basis of photosynthesis in plants , activates the isomerization of the retinal chromophore in rhodopsin culminating in animal vision , and converts provitamin D into vitamin D in our body . For technological and medical applications, the goal of understanding these and similar phenomena is the design of systems that function as well as nature itself, to produce photovoltaic current upon absorption of light , to label biological cells carrying diseases to the human body , or to produce light in a clean and efficient way via electroluminescence in organic light emitting diodes . Understanding and controlling the behavior of matter in all these situations require a deep knowledge of the fundamental quantum-mechanical laws governing the complex interplay of the molecules with light and with their environment at the microscopic scale over time scales ranging from femtoseconds to nanoseconds.