I will discuss observations of the effect of “microlensing" in quasars found in strong gravitational lens systems. This effect is produced by stars in the lensing galaxy which pass in front of one of the quasar images and work as small scale gravitational lenses. Observations of the microlensing provide a possibility to study the intrinsic structure of the “central engines” of quasars one the distance scales about the gravitational radii of the supermassive black holes powering these sources. I will also argue that quasar microlensing observations could provide a test of relativistic gravity theory in the strong field regime.
