Assuming a language with some inherent type safety (e.g., not JavaScript): Given a method that accepts a SuperType, we know that in most cases wherein we might be tempted to perform type testing to pick an action: public void DoSomethingTo(SuperType o) { if (o isa SubTypeA) { o.doSomethingA() } else { o.doSomethingB(); } } We should usually, if not always, create a single, overridable method on the SuperType and do this: public void DoSomethingTo(SuperType o) { o.doSomething(); } ... wherein each subtype is given its own doSomething() implementation. The rest of our application can then be appropriately ignorant of whether any given SuperType is really a SubTypeA or a SubTypeB. Wonderful. But, we're still given is a-like operations in most, if not all, type-safe languages. And that seems suggests a potential need for explicit type testing. So, in what situations, if any, should we or must we perform explicit type testing? Forgive my absent mindedness or lack of creativity. I know I've done it before; but, it was honestly so long ago I can't remember if what I did was good! And in recent memory, I don't think I've encountered a need to test types outside my cowboy JavaScript.