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.
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.