AbstractThis article reviews some of the fundamental scientific issues associated with solid inclusion compounds, and describes the approaches and strategies that may be used to investigate the structural, dynamic and chemical properties of these systems. Two particular families of solid organic inclusion compounds ‐ the urea and thiourea inclusion compounds ‐ are highlighted. In order to understand the fundamental nature of these solids, it has been necessary to apply a wide range of experimental, computational and theoretical approaches. Each technique provides information on a different aspect of the solid, and the combined information obtained from these complementary approaches allows a comprehensive understanding to be established. Several issues of contemporary interest for urea and thiourea inclusion compounds are described, and the approaches that have been taken towards a fundamental understanding of these systems are explai