An effective field theory (EFT) of a physical system is a theory of the dynamics of the system at energies small compared to a given cutoff. For some systems, low-energy states with respect to this cutoff are effectively independent of ("decoupled from") states at high energies. Hence one may study the low-energy sector of the theory without the need for a detailed description of the high-energy sector. Systems that admit EFTs appear in both relativistic quantum field theory (RQFT) and condensed matter physics. In some cases, the high-energy theory is known and the effective theory may be obtained by a process in which high-energy effects are systematically eliminated. In other cases, the high-energy theory may not be known, and the effective theory may then be obtained by imposing symmetry and "naturalness" constraints on candidate Langrangians. Many physicists currently believe that the Standard Model of particle physics is an example of such a bottom-up EFT. In both cases, the nature of the intertheoretic relation between an EFT and its (possibly hypothetical) high-energy theory is complex and arguably cannot be described in terms of standard accounts of reduction. This essay provides a review of this relation and what it suggests about the ontological status of EFTs and the extent to which the notion of emergence can be associated with them.