The tallest tree in the world is a coast redwood (Sequoia sempervirens), named Hyperion after a person in Greek mythology. He is no less than 115.55 m (379.1 feet) tall! This enormous tree was discovered only in August 2006 in a remote part of the Redwood National Park, California by naturalists Chris Atkins and Michael Taylor. Their first preliminary measurements were done with professional laser measurement equipment based on goniometry.
In September 2006 the tree was remeasured by Steve Sillett. This was done in the most accurate way: he climbed to the top of the tree to drop a tape from there straight to the ground. This tape drop was filmed for National Geographic.

In July 2006 some other record tall trees were discovered: "Helios" (named after the Greek God of the sun), the world's tallest known tree as of June 2006 (114.09 m), "Icarus" (113.14 m), and "Daedalus" (110.76 m).

The discoveries of 2006 are remarkable: these trees appeared to grow on slopes and not in the center of the valleys where water is most abundant, and where all the previous contenders to the title "tallest living tree in the world" grow. This means that it's not unlikely that there are even taller, not as such discovered trees out there, simply because people weren't looking for them there. Now the forested slopes are being sought trough by tall tree hunters Chris Atkins and Michael Taylor. It's on a location like that that in August 2006 the current tallest (known as such) tree of 115.55 m was found [2] [3].
In 2007 Chris Atkins and Michael Taylor had already scanned most of these new, unexpected locations. They think it's very unlikely a taller tree than Hyperion will be found, but you never know.

Hyperion has been quite lucky: only a few hundred feet from the base is the edge of a clearcut from the seventies. Clearcutting is a forestry practice in which all trees in an area are logged and the entire area is devastated. About two weeks before also Hyperion would have been attacked by the chain saws, this valley was added to the Redwood National Park during the Carter administration. Logging companies feared this would happen and worked 24/7 in the broad redwood valleys and kept logging old growth forests that were there long before men were ever entered these valleys.
In the seventies only an alarming 15% of the rich redwood forests remained, nowadays only 4% still exists and even today, as you read this, old growth Californian forests are being logged (more).

According to redwood standards, Hyperion is quite young and still growing vigourously. Sillett thinks the tree might be "only" 600 years old, which is about 20 years in human time [7].

Until July 2006 the tallest known Sequoia sempervirens was "The Stratosphere Giant" (image on the left). He is 112.83 m tall (measured in 2004, Steve Sillett) en was only discovered in August 2000 (by Chris Atkins) in Humboldt Redwoods State Park, California, where a lot of these giants grow. It is very difficult to appreciate the majestic height of these trees from a photo. The image on the left gives a little bit a sense of scale: on the right you can see a person hanging on a climbing rope (click to view a larger version).

Just as is the case with other giants, like the "The Federation Giant" (112 m), the exact location is kept secret by the park rangers to avoid a tourist stampede to the tree. This could unbalance the fragile ecosystem of the forest and could harm the tree directly: in the past things already went wrong for tall trees that became too popular (see below).

In case you don't have a clue about how tall 155.55 m (379.1 feet) is: the height of Big Ben in London is 96.3 m (316 feet), that of the statue of Liberty is 46.5 + 46.9 m (151 + 154 feet), so both tall constructions are largely lower than these trees.

Note that although the tallest living tree in the world is a coast redwood, the biggest living tree on the planet is a specimen of a related species, the giant sequoia. The largest giant sequoia is named "General Sherman".


Popularity
The tallest of these trees caught popular attraction for the first time in 1963, when the National Geographic Society organised an expedition in the redwoods forest. They found a tree of 111.9 they christened, very originally, the "Tall Tree"... Immediately, the tree became famous.

The leaf expansion decreases with increasing height [1]. Units are in meters.
This had its advantages and its disadvantages: the general public became aware of the uniqueness of these giants and the call for the creation of National Parks became louder. But fame also brought a flood of tourists. A road and a parking lot were constructed and buses started to bring loads of tourists, who all stood on the soft forest floor.
As you can see in the image on the right the size of the needles decreases with increasing height. The reason for this is that the tree has to do a bigger and bigger effort to get the water higher. Because of the tourists the ground around the tree was compacted and contained less and less water. As a result, already in the seventies, the top three to five meter started dying off. In 1990 the tree lost its top three meters and in 1991 he was not the tallest anymore. Currently this tree is known as the "National Geographic Society Tree" and is the fifth known tall Sequoia sempervirens.

In 1996 a taller tree was discovered and officially became the tallest tree in the world: the "Mendocino Tree" (111.4 m), growing in a remote valley with difficult terrain. The park rangers considered to keep the location secret to avoid damage to the tree and this is what happening with the in 2000 found "Stratosphere Giant" of 112.83 meter. The tree is still growing by the way. Only a couple of park rangers know which tree is the record one (the record trees stand in the midst of other similar trees).

It is absolutely uncertain if the tallest as such known Sequoia sempervirens is really the tallest Sequoia or the tallest Sequoia that grew in historical times. Indeed, it is very possible that in the logged parts of the original redwood forests (96%) there were taller specimens, who fell victim to the ever hungry chain saws.
Other species that can grow very tall are other conifers who also grow naturally on the northwestern coast of North America. Comes in second place: the Douglas-fir (Pseudotsuga menziesii) with 100.3 m (329 feet), followed by the Sitka spruce (Picea sitchensis), of which the tallest known specimen is 96,7 m (317,3 feet) tall. Just as the wood of coast redwoods, the wood of these trees is of excellent quality, so the once vast forests of these trees are almost completely gone. Without any doubt there were taller specimens in there. For example, in 1900 a Douglas-fir of 125.5 m (411.7 feet) was logged.

Tallest tree ever measured
The tallest tree ever measured was no Sequoia sempervirens, but a type of eucalypt (Eucalyptus), an Australian hardwood tree (images).
In 1872 forester William Ferguson spotted an extremely tall mountain ash (Eucalyptus regnans) near the Watts River, Victoria, Australia. Purportedly he was 132.6 m (435 feet) tall. In that time, multiple specimens of 140 m tall were reported in Tasmania.
We cannot check these measurements anymore: all these trees have been logged... Mountain ash was and is an important construction material in Australia.
Some reserve about these old measurements should be kept. The tallest mountain ash of which the heigth was measured with some certainty was 114.3 m tall and was also logged (1884).
Even nowadays the deforestation in mainland Australia and Tasmania is continuing. The relationship between environmental organizations and the state company Tasmania Forestry are tense, especially since they burned the biggest Australian tree "El Grande", although it was an accident according to Tasmania Forestry [4].
The tallest living eucalypts can be found in Tasmania. The tallest one, named "Centurion", is now 100 to 101 m (328 to 331 ft) tall and is the tallest living hardwood tree on earth. This eucalypt tree was only discovered in October 2008 by using a laser mounted on a plane, that was measuring the terrain height, forest height and forest biomass [5].
On the image on the right you can see the previous contender to that title, named "Icarus Dream". This tree has a height of 97 m (318.2 feet)