The development in recent years of ultrashort light sources in the attosecond regime has opened new avenues for the investigation of electronic and nuclear dynamics. In particular, the current development of UV-XUV or UV-X-ray pump-probe schemes with subfemtosecond temporal resolution represents a doorway to study chemical reactions in excited states of molecules in real-time, including biological reactions, such as e.g. those related to DNA damage and mutations. The aim of this project is to understand and ultimately control photochemical reactions in excited states of polyatomic molecules, especially relating to two fundamental processes in biology and chemistry: (i) cis-trans photoisomerization and (ii) the internal conversion of pi-pi*/n-pi* states in organic chromophores. Two top-notch experimental methods will be employed to achieve this goal: attosecond X-ray transient absorption (X-ATAS) at the University of California at Berkeley (United States), where the fellowship will be carried out under the supervision of Prof. S. Leone, and three-color femtosecond pump-probe velocity map imaging (fs-VMI) at Complutense University of Madrid (Spain), where the incoming phase will take place under the supervision of Prof. L. Bañares. The project is divided in three specific objectives and work-packages. Firstly, the powerful X-ATAS method will be employed to observe dynamics in real-time through individual carbon atom spectra and disentangle the dynamics underlying these processes in two polyatomic targets –nitroethylene and transbutadiene– and two bio-relevant ones –thymine and citosine. Secondly, attosecond control in real-time using X-ATAS on the pi-pi*/ n-pi* internal conversion in thymine will be performed in a pioneering experiment. Finally, effective femtosecond control with fs-VMI of these processes in nitroethylene and trans-butadiene, molecules of interest for technologies, will be performed based on the relevant results from the X-ATAS experiments.