Rogue waves, also known as freak waves, monster waves or extreme waves, are relatively large and spontaneous ocean surface waves that are a threat even to large ships and ocean liners. In oceanography, they are more precisely defined as waves whose height is more than twice the significant wave height (SWH), which is itself defined as the mean of the largest third of waves in a wave record. Therefore Rogue waves are not necessarily the biggest waves found at sea, they are, rather, surprisingly large waves for a given sea state. A wave, such as the one that probably caused the loss of the RV Ballena in Southern California, was a rogue wave that occurred in a rather calm sea state. Such a wave is nevertheless very small compared to the largest waves recorded by instrumented buoys, with significant wave height .

Once thought to be only legendary, they are now known to be a natural ocean phenomenon. Anecdotal evidence from mariners' testimonies and damages inflicted on ships suggested they occurred; however, their scientific measurement was only positively confirmed following measurements of the "Draupner wave", a rogue wave at the Draupner oil platform, in the North Sea on January 1, 1995. During this event, minor damage was inflicted on the platform, confirming that the reading was valid.

In the course of the Project MaxWave, researchers from the GKSS Research Centre, using data collected by ESA satellites, identified a large number of radar signatures that have been portrayed as evidence for rogue waves. Further research is under way to develop better methods of translating the radar echoes into sea surface elevation, but at present this technique is not proven .

Freak waves have been cited in the media as a likely source of the sudden, inexplicable disappearance of many ocean-going vessels. However, although this is a credible explanation for unexplained losses, there is to date little clear evidence supporting this hypothesis nor any cases where the cause has been confirmed, and the claim is contradicted by information held by Lloyd's Register. blank">Lloyd's Register–Fairplay The story that "200 large ships lost to freak waves in the past two decades" was published in _The Times (May 2006). The earliest reference seems to be in the press release by the European Space Agency (cited at the page bottom), and first quoted as "200 large ships of 600 ft long or more in the past two decades sunk without trace". At the time the claim was made, there had only been 124 ships of that size lost at sea in the time frame, according to Lloyd's Register, all with clear, known causes. The main culprits were the Iranian and Iraqi air forces in the 1980s; Iran-Iraq war. One of the very few cases in which evidence exists that may indicate a freak wave incident is the 1978 loss of the freighter MS München, detailed below. In February 2000, a British oceanographic research vessel sailing in the Rockall Trough west of Scotland encountered the largest waves ever recorded by scientific instruments in the open ocean. Holliday, NP, MJ Yelland, RW Pascal, VR Swail, PK Taylor, CR Griffiths, and EC Kent (2006). Were extreme waves in the Rockall Trough the largest ever recorded? Geophysical Research Letters, Vol. 33, L05613