The concept of an RNA world is a hypothesis firmly rooted in empirical data and is part of a long and complex scientific perspective that goes back more than fifty years to the discovery of the central role RNA and ribonucleotides play in protein synthesis and biochemical reactions took place. As the understanding of RNA biology progressed, several independent proposals of protein-free primordial life forms were suggested. Although this possibility was strongly reinforced with the discovery of ribozymes, there are many definitions of the RNA world, including several contradictory ones. One could say that it was an early, perhaps primordial, stage during which RNA molecules played a much more conspicuous role in heredity and metabolism and, particularly, in the origin and early evolution of protein biosynthesis. The overwhelming evidence for the catalytic, regulatory, and structural properties of RNA molecules, combined with their ubiquity in cellular processes, can only be explained with the proposal that they played a key role in early evolution and perhaps in the origin of life itself.