Better interoperability between systems, vocabularies, and organizations is considered necessary to most public organizations in order to better meet the demands from the users. The rapid growth of the Internet has been a driving force for both the user expectations and the enabling of such exchange. But succeeding with interoperability initiatives is hard, and the risks of failing are high, mostly because the expectations are too high and the inherent challenges are often underestimated. Many interoperability projects are over-specified and their findings are under-implemented.This paper discusses the challenges of interoperability in public sector and argues for a lightweight approach in order lower the gap between plans and reality. The Los system is illustrated as an example of this lightweight approach to interoperability.