In the past decade, human digital twins (HDTs) attracted much attention in and beyond digital twin (DT) applications. In this paper, we discuss the concept and the development of HDTs with a focus on their architecture, ethical concerns, key enabling technologies, and the opportunities of using HDTs in design. Based on the literature, we identified that data, model, and interface are three key modules in the proposed HDT architecture. Ethics is an important concern in the development and the use of the HDT from the humanities perspective. For key enabling technologies that support the functions of the HDT, we argue that the IoT infrastructure, data security, wearables, human modeling, explainable artificial intelligence, minimum viable sensing, and data visualization are strongly associated with the development of HDTs. Based on current applications, we highlight the design opportunities of using HDTs in designing products, services, and systems, as well as a design tool to facilitate the design process.