Membrane fusion is an important process for the survival of eukaryotes. Moreover, enveloped viruses enter the host cell utilizing membrane fusion. Therefore, it is essential to comprehend the mechanistic details of this intrinsic and crucial process. In this chapter, we will discuss several aspects of membrane fusion, including intermediate states and their energetics. We will further discuss the role of lipids in the stabilization of intermediate structures that lead to fusion pore opening. We will elaborate on the different existing models of membrane merger or fusion, with a major focus on the importance of lipid–peptide interplay for the completion of the process.