Book excerpt: This book, a consecutive contribution to the series Challenges and Advances in Computational Chemistry and Physics, focuses on understanding the photoinduced processes in biological systems. Understanding and fine control of light fate in molecules is vital for the progress of society and environmental safety. Light induced changes of various physico-chemical and spectroscopic properties in nucleic acids and proteins is the basis of fundamental biological events such as vision, DNA photodamage or photosensing. The investigation of these processes is challenging to both theoretical and experimental studies. This volume encompasses the quantum mechanics/molecular mechanics theory in several subfields, including: advanced computational methods for nucleic acids and proteins systems; dynamics, spectroscopic and physico-chemical properties of biological photoreceptors; DNA photodamage. This book is of interest to readers in both fundamental and application-oriented research by overviewing recent achievements in computational modeling of excited states in nucleic acids and proteins.