Abstract: Systems and methods for restricting code execution to a subset of language, runtime and instruction set capabilities. The subset of capabilities is preselected or inferred to ensure safety and stability. Furthermore, code execution can be monitored to guarantee the execution runs within the expected boundaries. By restricting execution to a safe subset of operations in a monitored or constrained execution environment, untrusted programs can be executed without the risk of adverse effects on the computing device or its software or network environment. Embodiments of the claimed subject matter can target various languages, runtime systems, platforms, and hardware.