Cell fusion has emerged as a powerful subject of debate in the last few years. Adult stem cell plasticity and the search for mechanisms to explain this process have led to the "rediscovery" of cell fusion. In nature, cell fusion is a normal process involved in sexual reproduction, tissue formation, and immune response. The recent observation that bone marrow derived cells fuse with several cell types introduces new and provocative questions. In this review, I shall recapitulate what is known about cell fusion and discuss its more controversial aspects. I shall highlight the most exciting open questions; its biological potential; pros and cons; and their implications on stem cell plasticity, regenerative medicine, and development.