Through the ages, scientific instruments have been used not only to advance understanding, but also to advance careers, dazzle audiences and impose standards. These 11 essays take stock of the philosophy of instrumentation and the impact of new instruments in both the physical and life sciences, carefully considering the important interplay between instruments and authority, audience and culture. Contributors include: Albert Van Helden on telescopes and authority; Jan Golinski on the demonstrative order of proof in Lavoisier's chemistry; Bruce J. Hunt on the development of electrical standards; Deborah Warner on terrestrial magnetism; Bruce Hevly on Stanford's supervoltage X-ray tube; Robert W. Smith and Joseph N. Tatarewicz on devices and black boxes; Thatcher Deane on the imperial astronomical bureau in the Ming dynasty; Thomas L. Hankins on Louis-Bertrand Castel's ocular harpsichord; Simon Schaffer on demonstration devices in Georgian mechanics; Timothy Lenoir on Helmholtz and the materialities of communication; and Robert Frank on instruments, biological techniques and the all-or-none principle.