Astronomy is entering a new observational era with the advent of several Large Telescopes, 8 to 10 metre in size, which will shape the kind of Astrophysics that will be done in the next century. Scientific focal plane instruments have always been recognized as key factors enabling astronomers to obtain the maximum performance out of the telescope in which they are installed. Building instruments is therefore not only a state of the art endeavour, but the ultimate way of reaching the observational limits of the new generation of telescopes. Instruments also define the type of science that the new telescopes will be capable of addressing in an optimal way. It is clear therefore that whatever instruments are built in the comming years they will influence the kind of science that is done well into the 21st century.

The goal of the 1995 Canary Islands Winter School of Astrophysics was to bring together advanced graduate students, recent postdocs and interested scientists and engineers, with a group of prominent specialists in the field of astronomical instrumentation, to make a comprehensive review of the driving science and techniques behind the instrumentation being developed for large ground based telescopes. This book is unique indeed in that it combines the scientific ideas behind the instruments, something at times not appreciated by engineers, with the techniques required to design and build scientific instruments, something that few astronomers grasp during their education.