Microelectromechanical systems (MEMS) include sensors and actuators that are fabricated with processes similar to those used in mainstream IC production. Automotive, biomedical, aerospace, and robotic MEMS devices can be produced with combinations of bulk and surface micromachining. The second half of the 20th century has seen information technology develop at an unprecedented rate. Beginning with the invention of the transistor in 1947, information technology has dramatically changed the manner in which modern society works and plays. Transistors led the way to ICs in the 1960s, and ICs enabled the development of virtually all commercial and consumer electronics products that are on the market today -- including personal computers, cellular telephones, CD players, and home video games. The past 50 years can be characterized by the ability to move electronic information from one place to another, with continuous improvements in efficiency, reliability, and costs. A similar revolution in information technology will occur in the first half of the 21st century, but the information conveyed will not be merely electronic. MEMS are the core technologies enabling the development of mechanical chemical, or biological "smart systems." At the heart of this revolution are two classes of instruments: sensors and actuators. Sensors are simply transducers that convert energy from one form to another (e.g., mechanical to electrical), and provide for passive measurement or monitoring. Actuators allow sensors to interact actively with the world. The ability to integrate sensors and actuators into efficient, reliable, and economic systems is fueling MEMS research in the US, Europe, and Japan. In very large scale integration (VLSI) processing, photo-lithographic techniques control the patterning of thin films and the deposition of dopants used to make transistor gates and metal contacts. MEMS processing uses these same techniques to create structural components that are essentially sub-millimeter-sized machine parts. These parts usually require post-fabrication processing or assembly in order to become workable devices. MEMS technology can generally be categorized into two groups: bulk and surface micromachining. These categories reflect not only different fabrication processes, but different post-fabrication techniques for finishing the mechanical subsystem. This article describes a handful of micromachined devices that are available, aswell as some future prospects at the research stage. Bulk and surface processing Bulk micromachining, one of the two major categories of MEMS processing, involves etching away selected portions of the substrate much like a sculptor in marble will start with a solid block and remove material until a final shape is created. Bulk micromachining typically etches away most of a silicon chip, with the remaining single-crystal silicon as the final structure. The two most common bulk etchants are wet-based potassium-hydroxide (KOH) and ethylene-diamine-pyrocatehol (EDP). Both etchants are anisotropic and thus remove material at different rates along different crystal planes to produce characteristic pyramidal pits and sloped sidewalls. KOH etching is incompatible with integrated active electronic devices "on-chip," since both potassium and hydroxyl ions contaminate the dielectric oxides that prevent conducting layers from shorting. In surface micromachining, structures are built on top of the wafer using thin films deposited through various standard methods familiar to IC fabrication. Unfortunately, the standard processes used to create electronic devices are not optimal for the creation of moving parts, so mechanical and electrical integration is more difficult than in bulk micromachining. For example, polysilicon-surface-micromachining processes typically use sacrificial oxide spacer layers that are etched away to render free polysilicon structures. This process flow is incompatible with standard IC processes that use oxides to isolate conductors. To create a complete electromechanical system, a hybrid arrangement of two separate processes is required, one for electronics and one for mechanical components. One compromise uses a standard CMOS process to fabricate electrical and mechanical parts, with an EDP bulk-machining process to remove portions of the substrate. The structures on top of the substrate are not affected by the bulk etch, provided they are sufficiently masked from the etchant. The oxide that separates and protects the electronics can be used to create beams and membranes over "pits" in the substrate, and these structures are free to move after release. This hybrid process can create many types of sensors that can be integrated with on-chip electronics for complete MEMS devices. Trade-offs between device function and manufacturing capability blur the division between MEMS device design and process development. Novel structures openthe door to new applications and show that the advantages of MEMS are inherent in the approach and not unique to any specific process. However, any nonstandard IC process has all the electronic integration problems associated with surface micromachining. Current commercial MEMS The accelerometer market is perhaps the most dramatic example of a business changed by micromachining. Many companies first venture into micromachining with accelerometers because there is a large market for small, cheap accelerometers in automotive air bag systems. A single, low-cost accelerometer can replace a network of crash sensors joined with an expensive wire harness. Design and fabrication approaches differ among companies: the Analog Devices series is surface micromachined in tensile polysilicon, the EG&G IC Sensors line is bulk micromachined out of multiple stacked wafers (Fig. 1), and Motorola's accelerometers are surface-micromachined polysilicon with a bulk-micromachined cap. Whatever fabrication approach is used, the goal is a small and inexpensive sensor that provides the desired functionality. Companies pursue micromachined accelerometers not because they are micromachined, but because they provide the desired function at a competitive cost. Siemens continues to build nonmicro-machined accelerometers by applying advanced manufacturing techniques to produce a cheap, reliable sensor with only six parts, but the market continues to shift toward micromachined solutions to reduce cost and size. The pressure