When I was younger, the largest telescope in the world was the 200 inch Hale Telescope located on Palomar Mountain in Southern California. At the time it was considered that a telescope with a 200 inch mirror was about the largest practical size that any telescope could be built. It was the largest telescope in the world for over 45 years.
Modern engineering, fabrication techniques and the invention of the computer have transformed the building of large telescopes. Recently permission was granted to begin building the Thirty Meter International Telescope on top of Mauna Keya on the Island of Hawaii, but because of the local opposition of a small group of Native Hawaiians, the actual construction has been delayed.
This gigantic telescope mirror will be made of 492 separate mirror segments that when combined into one large mirror will be 36 times the size of the 200 inch Hale telescope. It will be able to gather more light in less time and allow astronomers to see farther into space with clarity of up to 12 times the resolution of the Hubble Telescope.
Some of the goals of this telescope will be to observe and better understand the formation of planets and stars, and unlocking our understanding of galaxy formation as well as defining the overall structure of the universe.