My 2 Öre: First of all, I knew this was going to explode the conceptual network of the Boost community. We all pretend/think we know exactly what the difference is, and some of us have a fairly stable idea. None of us has a stable enough idea to formalize it into a communicable form. Alas, this is an important field of study, and for subsequent convergence. Having said that, I will nevertheless try to formalize my view of these two important notions. I agree with David A. regarding the (presumed) use of the construct being the vital discriminator, and that a policy (and a traits, in fact, which would make it a quite non-generic one...) does not have to be template-based. In my extremely humble opinion, a trait is strongly coupled to Feature Modeling in GP, providing a mapping from a Model for a particular Concept to the Model-invariant parts of the Features, i.e., placing a Model in a Feature Model. These invariant parts are mostly related types and constant values. One can then use these invariant pieces to get to the dynamic Features of the Concept. To be really convenient, the trait should cover all Models for a Concept, i.e., be generic (read "templates and specializations"...) A policy, on the other hand, is a (compile-time or runtime) behavioral alteration/add-on, not carrying any Concept-related information, but rather decorating a Model for a Concept. So, for me it is pretty easy, traits *only* describes Model-invariant parts of Concept Features while policies furnish Models of Concepts. Unfortunately, the words have been used more or less interchangeably in the early GP days of C++. We here have a golden opportunity to change that... /David