Solar activity is the ultimate driver of Space Weather. From its various manifestations, coronal mass ejections (CMEs) have the most severe impacts on ground- and space-based technological systems. CMEs are massive eruptions of magnetically confined plasma within structures that can stay stable for up to several solar rotations and be suddenly ejected into the heliosphere. There is a long chain of processes leading to these highly dynamical phenomena, from the generation of the solar magnetic field at the bottom of the convective zone, the emergence of magnetic flux tubes through this inner solar layer, the interaction of the emerged flux with the surrounding coronal magnetic field, and, finally, the destabilization of the coronal structure and CME initiation. Understanding the physics behind these processes is fundamental to improve our capability to mitigate space weather impacts. Session 5 is devoted to review our current understanding, from observational, theoretical, and modeling points of view, of some key aspects in the chain of processes leading to CMEs.