 The problem of replication and reproduction arises out of the history of genetics [see the entry  gene  for a historical review]. It is tied to the concept of the gene and its generalization in an evolutionary context [see the entry  evolution].  Richard Dawkins introduced the notion of replicators—things that self-replicate—as a universalization of evolutionary understandings of genes. Dawkins argued that replicators are the sine qua non of evolution by natural selection [see the entry  natural selection],  while other accounts only require reproduction as one of its defining features. What exactly is a replicator? How are replicators different from genes? Can evolution by natural selection occur without the existence of replicators? Besides the biological domain, are there any other domains in which replicators have been postulated? To answer these questions, we will first provide some background for Dawkins’ notion of the replicator and its ties with the concepts of the gene and information. We will then introduce the distinction between replicators and vehicles in the context of biological evolution, followed by the extension of this to other domains. Finally, we will discuss some of the challenges to the idea that replicators are necessary conditions for evolution by natural selection.