AAO Colloquium. Thursday, 23rd November - 3:30pm AAO Conference Room Gravitational Lensing as a versatile astrophysical tool Joachim Wambsganss Univ. of Melbourne and Potsdam University In the two decades since the discovery of the first double quasar, gravitational lensing has been applied in quite a number of astrophysical fields and has developed into a very useful astrophysical and cosmological tool. After a few historic remarks, the basics of lensing will be briefly reviewed. In the main part of the talk, three recent applications of lensing in astrophysics/cosmology will be presented, including the determination of the Hubble constant from the measurement of the time delay in multiple quasars, microlensing of quasars and its constraints on quasar size and dark matter objects, and the search for extrasolar planets with gravitational lensing. A few comments on other recent detections of weak and very weak lensing effects will conclude the talk.