With the worldwide adoption of the Internet as a ubiquitous platform for exchanging information, new opportunities are available for sharing educational experiences and material. Thousands of institutes worldwide offer access to courses, lessons, seminars, and exercises, typically stored in repositories and offered to users under a variety of formats. Due to their educational nature, these contents are often referred to as Learning Objects. This article describes the "ProLearn Query Language", a query language that we have developed for repositories of learning objects. PLQL is primarily a query interchange format, used by source applications (or PLQL clients) for querying repositories (or PLQL servers). In defining PLQL, we have combined exact search, used for selecting learning objects by means of queries on their metadata, and approximate search, used for locating learning objects by means of keyword-based search. A query in PLQL contains both "exact clauses" and "approximate clauses", where each clause is syntactically well-defined; exact clauses match keywords against metadata with a known structure, while approximate clauses match keywords freely, either against metadata or against indexes based upon the documents' contents. In this article, we give a precise description of the semantics of PLQL, concerning both kinds of clauses and their mutual relationship and describe two experimentation efforts around PLQL: one involving the ARIADNE repository and the other the EUN Learning Resource Exchange initiative