Concurrency is a crucial aspect of modern software development, enabling efficient utilisation of multi-core processors and the development of responsive and scalable applications. First-class user-level threads, also known as green threads, are an approach to concurrent programming where thread management is handled entirely at the user level, without relying on the operating system’s threading model. In this paper, we present a systematic literature review (SLR) that explores the current state of research and practice in the area of firstclass user-level threads. We review a wide range of studies, including research papers, technical reports, conference proceedings, and industry reports, to identify the key concepts, techniques, benefits, and limitations of first class user-level threads. We also analyse the trends and challenges in this area, highlight the applications and domains where first-class user-level threads are commonly used, and provide insights into future research directions. Our SLR aims to provide a thorough examination of the cutting-edge advancements in the field of first-class user-level threads, serving as a valuable resource for researchers, practitioners, and developers interested in concurrent programming and thread management.