A systematic review and analysis of the literature of diffuser-augmented horizontal-axis turbines are presented. A collection of 155 articles in the area is analyzed and classified. The work sample is divided into 16 main research branches for discussion. Performance assessment metrics are proposed based on power coefficient and tip-speed-ratio, to quantify and compare all diffuser-augmented turbines in a unified, meaningful manner. Design suggestions for the development of diffuser-augmented turbines are pointed out based on the analysis of 73 cases. A power coefficient assessment on the work sample presented that, in 58% of the cases, the diffuser-augmented turbines surpassed the power coefficient of scaled bare turbines of the same diameter. A tip-speed-ratio assessment presented that almost 90% of the diffuser-augmented turbines developed a narrower operational interval. Five high-performing diffuser-augmented turbines are discussed, highlighting their methodologies and contributions. Caution is advised when coupling a diffuser to a bare turbine with an already high power coefficient; the diffuser-augmented turbine, especially in those cases, should be designed employing a simultaneous diffuser-rotor optimization.