Aero engine design simulation technology is gradually developing toward integration and coupling, accompanied by improvements in system engineering methodology, and the field of aero engines is currently undergoing a digital transformation. This article reviews the concepts, current status, needs, and key technical issues of aero engine digitalization, with the aim of providing a reference and support for the digital transformation of aero engines. Given the complexity of aero engines, the concepts and substantive connotations of model-based system engineering, digital twins, and digital threads are analyzed, and a definition of digital engines is developed. The use cases and development status of digital engines in China and other countries are summarized, and the work that needs to be carried out to address technical trends and challenges, such as model algorithm libraries, general platforms, data connectivity and processing, coupled integrated simulation technology, data analysis and decision-making, is discussed. The content and directions of research in China and other countries on the key technical issues of digital transformation, such as discipline simulation integration methods, system model and discipline model connection methods, and digital twin modeling methods, are discussed. The ideas and specific research content for digital engines can provide a basis for subsequent planning and construction.