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.