After looking at the details of ring laser design and operation and putting it into perspective with alternative sensor designs, we now look at the actual areas of application of large ring laser gyroscopes. We start off by discussing ring resonator performance from the viewpoint of an optical frequency source. Then we carry on by summarizing how the application of large ring lasers to high resolution rotation sensing has enabled the research field of rotational seismology, before discussing the achievements obtained with ring lasers for space geodesy over the years. Finally we briefly touch the field of the highly sensitive detection of non-reciprocal phenomena, such as magneto-chiral birefringence, before addressing the applicability of ring laser gyroscopes to tests in fundamental physics. The book concludes with an outlook on the future of large ring lasers.