As we connect more microcontrollers to the Internet and employ them to control the physical world around us, their reliability and security are increasingly important. Many microcontrollers provide limited facilities for hardware isolation, and real-time OSes offer custom APIs, that require coupling applications into the ecosystem and abstractions of that specific OS to leverage isolation. This article investigates the use of software sandboxing of applications to support isolation for resource-constrained devices. Toward this, we detail the design of eWASM, a processes abstraction that adapts a popular sandbox, Wasm, for microcontrollers. eWASM provides a runtime to constrain memory accesses and control flow, enabled by our aWsm Wasm compiler. We discuss and evaluate its multiple implementations that effectively trade time and space, optimizing for the constraints of embedded systems. This enables popular languages (e.g., C) to be effectively sandboxed by software. We demonstrate performance within 40% of native C on Polybench. We believe this is a practical and compelling result for many IoT domains, and it represents the first compiled sandboxing environment for microcontrollers. We show that restrictions of the current Wasm specification lead to significant memory consumption and provide suggestions for the creation of an embedded-specific Wasm variant.