The need for an ontological layer on top of data, associated with advanced reasoning mechanisms able to exploit ontological knowl-edge, has been acknowledged in the database, knowledge representation and Semantic Web communities. We focus here on the ontology-based data querying problem, which consists in querying data while taking on-tological knowledge into account. To tackle this problem, we consider a logical framework based on existential rules, also called Datalog ± . In this course, we introduce fundamental notions on ontology-based query answering with existential rules. We present basic reasoning techniques, explain the relationships with other formalisms such as lightweight de-scription logics, and review decidability results as well as associated al-gorithms. We end with ongoing research and some challenging issues.