Static types can and usually are checked at compile time. Whether the language runtime also uses type checks while running depends entirely on the language and

Static types can and usually are checked at compile time. Whether the language runtime also uses type checks while running depends entirely on the language and the runtime. IIRC, the JVM (to take an example) more or less throws away a lot of the static type information -- which is often too generic to be useful in guiding optimization -- in favor of runtime profiling, and several of its key optimizations involve inserting runtime type checks.