Ontology is a significant research theme in GIScience. While some researchers believe that the progress in GIScience is being directed through an engagement with the concept of ontology, some dismiss it as irrelevant. This paper is aimed at (i) exploring the theoretical and practical roles of ontologies; (ii) making the definitions and terminology explicit; (iii) assessing the applicability of ontology to problems in the geographical domain; and (iv) assessing whether a unified approach to ontology exists in GIScience. The results will be helpful for GIScientists in (i) understanding the validity of employing ontology within their own work, (ii) assessing what operational framework of terms and methods to use for developing their own ontology, and (iii) to assess what existing ontological models are available and applicable within their domain or application. A comprehensive and critical review will also help in identifying the signficant issues and directing future research agenda in GIScience.