The term electrowetting was first introduced in 1981 to describe an effect proposed for designing a new type of display device [G. Beni and S. Hackwood, Appl. Phys. Lett. 38, 4, pp.207-209, 1981]. The electrowetting effect was originally defined as "the change in solid electrolyte contact angle due to an applied potential difference between the solid and the electrolyte". Electrowetting is akin to but distinct from electrocapillarity. In the past 25 years or so a large number of devices based on electrowetting have been devised. In particular, electrowetting has recently been used successfully as one of several techniques used to actuate microdroplets in a digital microfluidic device. Electrowetting is essentially a phenomenon whereby an electric field can modify the wetting behavior of a droplet in contact with an insulated electrode. If an electric field is applied non-uniformly then a surface energy gradient is created which can be used to manipulate a droplet sandwiched between two plates. Electrowetting arrays allow large numbers of droplets to be independently manipulated under direct electrical control without the use of pumps, valves or even fixed channels. Electrowetting is now used in a wide range of applications from modulab to adjustable lenses and electronic displays.