Since long, man has used clocks and watches to measure time. But those were crude watches and did not measure time accurately. A few years ago, scientists were able to develop a very sophisticated clock known as ‘atomic clock’. With its development, a new era has been ushered in the field of time measurement. It is a wonder clock, that remains accurate to one second for 1,700,000 years. Today we have mainly three types of clocks and watches — mechanical, electrical and electronic. Mechanical clocks and watches are spring-driven, electrical clocks are battery-powered and the electronic ones are quartz-based. All these clocks and watches show time quite accurately. But if they run continuously for long periods, they can get slow or fast. Now, the smallest internationally accepted unit of time is the atomic second. It is based on atomic clock. Prior to this, the second was the standard of time which was measured as a portion of Earth’s rotation as 1/86400th of a day. An atomic clock uses the frequencies produced by atoms or molecules. The time is measured by counting the number of vibrations. Most of the atomic clocks make use of frequencies in the microwave range from about 1400 - 40,000 Megahertz. In 1947, an oscillator controlled by frequencies of ammonia molecule was constructed. An ammonia-controlled clock was built in 1949 at the National Bureau of Standards, Washington D.C. In 1955, a caesium-beam atomic clock of high precision was first put in operation at the National Physics Laboratory, Teddington, England. After that a number of laboratories started producing commercial models of caesium-beam atomic clocks. In the caesium clock, the caesium is heated in a small oven. The caesium produces a beam which is directed through an electromagnetic field. The 5 Megahertz output from a quartz clock is multiplied to give 9192631770 Hertz that controls the electromagnetic field. Part of the 5 Megahertz output is used to derive a clock display unit which indicates time. In the recent years, some other atomic clocks have also been developed which make use of ammonia maser, hydrogen maser and rubidium gas cells. Atomic clocks of 1960s were very large in size but by 1978 their sizes have been sufficiently reduced to fit in a small box. Atomic clocks are used as standard to time. They are also being used in some sophisticated navigation systems and deep space communication.