The Dynamic Execution Layer Interface (DELI) offers the following unique capability: it provides fine-grain control over the execution of programs, by allowing its clients to observe and optionally manipulate every single instruction-at run time-just before it runs. DELI accomplishes this by opening up an interface to the layer between the execution of software and hardware. To avoid the slowdown, DELI caches a private copy of the executed code and always runs out of its own private cache.In addition to giving powerful control to clients, DELI opens up caching and linking to ordinary emulators and just-in-time compilers, which then get the reuse benefits of the same mechanism. For example, emulators themselves can also use other clients, to mix emulation with already existing services, native code, and other emulators. This paper describes the basic aspects of DELI, including the underlying caching and linking mechanism, the Hardware Abstraction Mechanism (HAM), the BinaryLevel Translation (BLT) infrastructure, and the Application Programming Interface (API) exposed to the clients. We also cover some of the services that clients could offer through the DELI, such as ISA emulation, software patching, and sandboxing. Finally, we consider a case study of emulation in detail: the emulation of a PocketPC system on the Lx/ST210 embedded VLIW processor. In this case, DELI enables us to achieve near-native performance, and to mix-and-match native and emulated code.