Abstract : The use of constraints in types is quite natural. Yet, integrating constraint based types into the heart of a modern, statically typed, object-oriented programming language is quite tricky. Over the last five years we have designed and implemented the constrained types framework in the programming language X10. In this paper we review the conceptual design, the practical implementation issues, and the many new questions that are raised. We expect the pursuit of these questions to be a profitable area of future work.