The structure and composite nature of a learning object is still open to interpretation. Although several theoretical studies advocate integrated approaches to the structure and aggregation level of learning objects, in practice, many content specifications, such as SCORM, IMS Content Packaging, and course authoring tools, do not explicitly state the aggregation level or granularity of learning content. The aim of this paper is to review, compare, and amalgamate different content models for learning objects into a single and coherent learning content hierarchy. To fulfil the objectives of this study, a substantive body of literature was reviewed and analysed to identify current issues in the field of learning objects, and more specifically, learning object content models. (Contains 3 figures and 3 tables.)