Historically, the discovery of tools, or evidence that tools have been used, has been taken as proof of human activity; certainly the invention and spread of new tools has been a critical marker of human progress and has increased our ability to observe, measure, and understand the physical world. In astronomy the tools are telescopes and the optical and electronic instruments that support them. The use of the telescope by Galileo marked the beginning of a new and productive way to study and understand the universe in which we live. The effects of this new tool on what we can see, and how we see ourselves, are well known. However, after almost four centuries of developing ever more sensitive and subtle instruments as tools for astronomy, it might have been expected that only a few minor improvements would remain to be made, or that possibly the law of diminishing returns would have taken effect. On the contrary, the new instruments and ideas for new instruments described in this book make it clear that the rate of progress has not diminished, and that this subject is still as exciting and productive as ever. Instrumentation for Ground-Based Optical Astronomy was chosen as the theme for the Ninth Santa Cruz Summer Workshop in Astronomy and Astrophysics.