  Digital Twin technology is an emerging concept that has become the centre of
attention for industry and, in more recent years, academia. The advancements in
industry 4.0 concepts have facilitated its growth, particularly in the
manufacturing industry. The Digital Twin is defined extensively but is best
described as the effortless integration of data between a physical and virtual
machine in either direction. The challenges, applications, and enabling
technologies for Artificial Intelligence, Internet of Things (IoT) and Digital
Twins are presented. A review of publications relating to Digital Twins is
performed, producing a categorical review of recent papers. The review has
categorised them by research areas: manufacturing, healthcare and smart cities,
discussing a range of papers that reflect these areas and the current state of
research. The paper provides an assessment of the enabling technologies,
challenges and open research for Digital Twins.
