The wide range of applications of two-phase natural circulation loops (NCLs) demands intense analyses of boiling systems, and consequently a large number of experimental and theoretical research findings are available in the literature. This comprehensive review focuses on the state-of-the-art appraisal of the related technology, with a primary focus on studies carried out in the last two decades. General modeling strategies have been reported and compared. Different forms of thermal-hydraulic instabilities appearing in two-phase NCLs have been summarized, with discussions on possible ways of identification and their differences. In addition, studies on system instability are presented in a categorywise grouping following a chronological style in terms development of knowledge. Coupled nuclear instabilities are reported as well in some detail. Finally, this review identifies the gray areas where the information is not quite complete and suggests the required path of future research.