Photosynthetic plants and bacteria convert the solar energy into chemical energy with almost perfect efficiency, but the detailed molecular-level mechanism of such conversion has remained unknown for decades. Thanks to recent developments in X-ray crystallography and ultrafast nonlinear spectroscopy, important pieces of structural and dynamical information are now available for many key components of the photosynthetic unit. The complexity and disorder present in these kinds of light harvesting systems are substantial. New theoretical models are needed in order to explain all existing spectroscopic data in a consistent manner. We currently have a successful quantum mechanical model for the light harvesting complex 2 (LH2) of purple bacteria, which has been tested against various spectroscopic data. Further development of this model and the explanation of existing and new spectroscpic data can help to uncover the great mystery of photosynthesis.