Biological Clocks Virtual Museum - This exhibit explores the inputs, outputs, and mechanisms of biological clocks with a primary focus on circadian rhythms.

Cesium Atomic Clocks - Kinds of cesium clocks, with pictures, plus a description of their operation and the science behind them.

Doug's Atomic Clocks - Details on Doug Hogarth's experiments with his atomic clock.

Green Tyme - Using a patented KinderClock Chronichron, this site simultaneously displays both standard time's fixed hours and natural time's variable lengths of days and nights.

How Stuff Works: Atomic Clock - Concise history, background, definitions and operational information regarding atomic clocks.

How Stuff Works: Pendulum Clock - Concise description using drawings and animation to show the operation of a pendulum clock.

Smithsonian Institute - On Time - History of the clock and how Americans have measured time and been affected by it.