Ontology–based information extraction is considered as an effective method to improve the performance of information extraction (IE) systems. For research and disbursal of customized healthcare services, a major challenge is to efficiently retrieve and analyze the individual patient data from a large volume of heterogeneous data over a long span of time. This requires effective ontology-based information retrieval approaches for clinical information systems. This book is an attempt to highlight the key advances in ontology-based information retrieval techniques especially in the healthcare domain. The varied chapters attempt to uncover the current challenges in the application of ontology-based information retrieval techniques to the healthcare systems. This book is the first of its kind that highlights the ontology-driven information retrieval mechanisms and techniques being applied to healthcare as well as clinical information systems. It can serve as a textbook for courses in healthcare systems. It can also serve as a reference book to medical practitioners and researchers involved in implementing as well as providing customized health care solutions to patients.