Syntactic type soundness is too weak to tell apart different ways of running a program that mixes typed and untyped code. Complete monitoring is a stronger property that captures a meaningful distinction — a language satisfies complete monitoring iff it checks all interactions between typed and untyped code.

The literature on mixed-typed languages presents (at least) three fundamentally different ways of thinking about the integrity of programs that combine statically typed and dynamically typed code. Recently, we have been sorting them out.