The central engines of quasars are of central importance in the fundamental physics of black hole growth, and in our understanding of galaxy evolution. They are the sites 
of most rapid black hole growth, and this growth is believed to underpin the evolution of the most massive galaxies through AGN feedback. However, their small angular size 
means they will remain off-limits to optical telescopes for the foreseeable future. I will discuss recent developments in the field of gravitational microlensing (by stars 
in the lensing galaxy) that afford us a probe of the micro-to-nano arcsecond structure of a small number of quasars. Although the technique is limited in terms of its 
current sample, it will come into its own with the advent of LSST  and the many thousands of new lensed systems it will discover. I will highlight the important discoveries 
made already using the technique, regarding the size and structure of the continuum and broad line emission regions. I look to the future of LSST and the ELT's, and what 
the technique will reveal through them, as well as its significance for the GAIA mission.