The Loch Ness Monster, sometimes called "Nessie" or "Ness" (Scottish Gaelic: Niseag) is a mysterious and unidentified animal or group of creatures said to inhabit Loch Ness, a large deep freshwater loch near the city of Inverness in northern Scotland. Nessie is usually categorized as a type of lake monster.

Along with Bigfoot and the Abominable Snowman, Nessie is one of the best-known mysteries of cryptozoology. Most mainstream scientists and other experts find current evidence supporting Nessie unpersuasive and regard the occasional sightings as hoaxes or misidentification of mundane creatures or natural phenomena.

Rumours of a monster or animal living in the loch are claimed by believers to have been known for several centuries, though others have questioned the accuracy or relevance of such tales, which were generally unheard of before the 1960s when a strong wave of interest in legitimizing Nessie's 1930s-based history began. The earliest report is taken from the Life of St. Columba by Adamnan, recorded in the 6th century. It describes how in 565 Columba saved the life of a Pict, who was being supposedly attacked by the monster. Adamnan describes the event as follows:

"...(He) raised his holy hand, while all the rest, brethren as well as strangers, were stupefied with terror, and, invoking the name of God, formed the saving sign of the cross in the air, and commanded the ferocious monster, saying, "Thou shalt go no further, nor touch the man; go back with all speed." Then at the voice of the saint, the monster was terrified, and fled more quickly than if it had been pulled back with ropes, though it had just got so near to Lugne, as he swam, that there was not more than the length of a spear-staff between the man and the beast. Then the brethren seeing that the monster had gone back, and that their comrade Lugne returned to them in the boat safe and sound, were struck with admiration, and gave glory to God in the blessed man. And even the barbarous heathens, who were present, were forced by the greatness of this miracle, which they themselves had seen, to magnify the God of the Christians".

Critics have questioned the reliability of the Life, noting a different story in which Columba slays a wild boar by the power of his voice alone . They also point out that the event is said to have occurred on the River Ness, not in the Loch, and that Adamnan reports Columba encountering and conquering assorted "monsters", at various places in Scotland, throughout his "life". Additionally, they point out that the Loch Ness monster has no other reported instance of attacking anyone, and in fact is generally portrayed as shy.

The first modern sightings, which made Nessie a widespread sensation, occurred on May 2, 1933. The newspaper Inverness Courier carried a story of Mr. and Mrs. John Mackay, who reportedly saw "an enormous animal rolling and plunging on the surface." The report of the "monster" (a word chosen by the editor of the Courier) became a media sensation with London papers sending reporters to Scotland, and a circus even offering a reward of £20,000 for capture of the monster. It was at this point that the belief in a monster-inhabited Loch Ness first began to take form.

Further preoccupation with the Loch Ness Monster was aroused by a photograph allegedly taken by surgeon R.K. Wilson on April 1 (All Fool's Day), 1934, which seemed to show a large creature with a long neck and head lifted above the water. Decades later, on March 12, 1994, Christian Spurling admitted to having faked the photo after being hired by the Daily Mail to track down Nessie (the photo had by that time been printed worldwide as "absolute evidence"). Spurling also stated that Dr. Wilson did not take the photo, and his name was only used to give an added air of credibility.

Many people still alive today claim to have seen the monster, including one man working for a conservation agency, who saw a "very large animal, covered all in scales" coming at him. The man saw the alleged monster while scuba diving near the middle of the lake. There is a false rumour circulating that the last sighting was in 1997 by a group of school children, but many people claim to have seen Nessie many times in the years since the new millennium began.

Regardless of whether anything is actually in the loch, the Loch Ness Monster has some mythological significance to the people of the town.

Most accounts of Nessie's appearance, including historical ones, indicate a creature resembling the long-extinct plesiosaur. Actual fossil evidence for this Mesozoic creature shows it to have been physically large, with a long neck and tiny head, with flippers for propulsion. The alleged connection of this creature with the Loch Ness monster has made it a popular topic in the field of cryptozoology. However, most scientists suggest the idea that the Loch Ness Monster is a remnant of the Mesozoic era is highly unlikely; there would need to be a breeding colony of such creatures for there to have been any long-term survival, and coupled with the fact that plesiosaurs needed to surface to breathe, this would result in far more frequent sightings than have actually been reported (though some animals, such as crocodiles, that need to breathe air intermittently, can stay underwater for extended periods of time by remaining still and conserving their oxygen supply). Many biologists also argue Loch Ness is not large or productive enough to support even a small family of these creatures. Moreover, the loch was created as the result of geologically recent glaciation and was frozen solid during recent ice-ages. Another ancient creature that has been suggested as looking like descriptions of Nessie is a Zeuglodon. The Zeuglodon, also known as a Basilosaurus, was an ancient Cetacean from which modern whales are believed to be descended. The Zeuglodon actually more closely resembled a large, aquatic serpent, such as many eyewitnesses have described Nessie.

Other sightings, however, do not fit the plesiosaur description or even a water-bound creature: In April 1923, Alfred Cruickshank claimed to have seen a creature 3 to 3.5 metres (10 to 12 ft) long, with an arched back and four elephant-like feet cross the road before him as he was driving. Other sightings report creatures more similar to camels or horses.Yet others have come to the conclusion that Nessie is a giant worm or shell-less mollusk, resembling a giant slug. The Spicer sighting (where a Scottish couple observed a worm-like creature crossing a road and then plunging into the lake) of 1934 supports this view.

Theories as to the exact nature of the Loch Ness Monster sightings are varied: pareidolia or misidentification of seals, fish, circus elephants, logs, mirages, seiches, and light distortion, crossing of boat wakes, or unusual wave patterns. Very large sturgeon have been found in inland streams close to Loch Ness, and due to the sturgeon's size and unusual appearance, one could easily be mistaken for a monster by someone not familiar with it. A recent theory postulates that the "monster" is actually nothing more than bubbling and disruptions in the water caused by minor volcanic activity at the bottom of the loch. This latter argument is supported to a minor degree by a correlation between tectonic motion and reported sightings.

Another theory is that it was in fact an elephant in the water, with only its trunk visible. Records indicate that there was a traveling circus in town the day that the famous "Surgeon's Photo" was taken

Some researchers, notably John Keel, F.W. Holiday, and Erik Beckjord, postulate that there are no unknown physical creatures within the loch. Because of the absence of physical evidence, these researchers argue that many of the reported sightings can be attributed to hoaxes or misidentification of conventional creatures and objects. However, they also argue that a residue of reported Loch sightings could be paranormal, or supernatural in nature, i.e., having a temporal semi-transparent construction, similar to other anomalous phenomena such as ghosts, Bigfoot, UFOs and demons.

Other researchers claim that amorphous images of the Loch Ness Monster have been caught on film. For example, in 1983, a small team from the Nessie Research Project took 83 seconds of 16 mm colour film of a white, ten foot long, amorphous, shape-shifting creature that had strange heads on its back, matching in many respects the equally strange 1967 35 mm b/w Raynor Film, taken by resident Dick Raynor, formerly of the Loch Ness Investigation Project (now defunct). The 1983 film was shown to the 1987 International Society of Cryptozoology convention in Edinburgh, at the Royal Museum, where co-chair Dr. Jack Gibson declared it to be the "Best Nessie video he had ever seen".

The NRP team went to Loch Ness that summer, and with the assistance of psychic Alex Crosbie, obtained eight still photos, (7 in 35 mm, 1 in 110) of a 48 foot (15 m) long flexible telephone pole-like object, with a "shield-shaped head" (Dr Bruce Maccabee, film analyst). The photos showed the creature interacting with several yachts, moving to avoid them.