When Galileo first turned a tiny telescope on the sky in 1609, the human view of the universe was changed forever. Over the following centuries astronomers built bigger telescopes, until for most of the 20^th century two large telescopes led in optical astronomical discovery: the 2.5-m (“100-inch”) Hooker Telescope and the 5-m (“200-inch”) Hale Telescope – both in Southern California. Then, in the last 30 years, new technology has led to a plethora of telescopes with 8 to 10-m diameter mirrors that have sprouted on remote mountains. The biggest of all is the enormous 39-m European Extremely Large Telescope (ELT) now under construction in Chile at a cost €1.5 billion. This lecture will look at how optical telescopes work, the people who built them, and some of their mind-boggling discoveries. New technology has now revolutionised their construction, operation and scientific abilities. The latest telescopes are among the most sophisticated machines ever imagined and built.