The Web puts a huge number of learning resources within reach of anyone with Internet access. However, many valuable resources are difficult to find due to the lack of interoperability among learning repositories. In order to achieve interoperability, implementers require a common query framework. This paper discusses a set of methods referred to as Simple Query Interface (SQI) as a universal interoperability layer for educational networks. The methods proposed can be used by a source for configuring and submitting queries to a target system and retrieving results from it. The SQI interface can be implemented in a synchronous or an asynchronous manner and is independent of query languages and metadata schemas. In this paper SQI's universal applicability has been evaluated by more than a dozen implementations demonstrated in three different case studies. SQI has been finalized as a standard in the CEN/ISSS Learning Technologies Workshop. Latest developments of SQI can be followed at http://www.prolearn-project.org/lori/. (Contains 3 figures and 1 table.)