Abstract: Common envelope evolution (CEE) is a critical step in the evolution of binary stars. During the unstable mass transfer, the interaction of the secondary star with the primary's (donor) envelope allows it to expel the envelope's gas and tightens the binary's orbit, resulting in a compact binary configuration. This channel is necessary for the formation of numerous exciting astrophysical phenomena: X-ray binaries, supernova progenitors, sources of ionizing radiation, or progenitors of gravitational wave sources. Nevertheless, the phase CEE is also the least known, as its complexity prevents a monolithic theoretical approach. Hence, different theoretical models and simulations usually focus on one part of the problem. In order to connect all these heterogeneous approaches, observations are required. Within the last three decades, a new class of astrophysical transients called Luminous Red Novae (LRNe) has been associated with the merger of massive stars, providing new observational clues to the physics occurring at the onset of the common envelope. In my talk, I will review the major challenges in CEE and how observations of LRN can help to advance in this field.