The cell is the primary active biological ingredient in TE/RM. In this chapter, we examine the principal classes of stem and progenitor and committed cell populations currently under active commercial development for application in TE/RM. We highlight what features of these cell populations make them amenable to successful commercialization and discuss the possible mechanisms of action by which they present therapeutic potential. Additionally, we point out aspects of their phenotype and function that may impede their successful negotiation of the regulatory approval process. Throughout, we illustrate these concepts with specific examples of products currently under consideration by the US Food and Drug Administration.