Gravitational Lenses and VLBI

The earliest VLBI observations of gravitational lenses (1979-80) quickly demonstrated the power of VLBI to resolve lensed images and provide important model constraints on the lensing mass. I summarise the ways in which VLBI observations have contributed to our knowledge and understanding of lens systems, not only through the detection of multiply-imaged substructure on mas-scales, but also by their ability to probe the conditions in the ISM of (high-redshift) lensing galaxies via effects such as Faraday rotation, depolarisation and scatter-broadening. I conclude with a consideration of what the future holds for VLBI observations of lens systems.