This paper discusses common iteration schemes and highlights the interest of using explicit iterators. The advantages of external iterators are compared to those of internalized iterators. The integration of an iterator class hierarchy to an existing library without modifying the latter is detailed. This integration brings an extra level of abstraction to the library, which thus becomes more flexible, more adapted to certain design patterns and hence can be used in a higher-level way. Such an integration is not only possible, but can even be done in an optimized way, taking into account the specific structure of the collection traversed. A slight extension of existing class libraries can also be implemented that does not cause any compatibility problem and does not break existing code, but allows even further abstraction and makes it easier for the developer to use high-level, optimized, external iterators.