A trawler is a fishing vessel designed for the purpose of operating a trawl, a net that is dragged along the bottom of the sea (or sometimes above the bottom at a specified depth). Modern "rock hopper" nets impliment heavy rubber wheels that allow the net to crawl over rough ocean bottoms without tearing on rocks.

Sailing trawlers were limited to trawling at depths of 55-75 metres, while modern trawlers often trawl to 900 metres, with experiments having gone even deeper. Sizes of modern trawlers vary, but range up to vessels of 2,500 to 3,000 tons.

Refrigeration machinery aboard allows trawlers to stay out for several weeks filling the hold.

One of the biggest ports for trawlers was the city of Kingston upon Hull (or simply 'Hull') in Yorkshire on England's north-east coast.

During World War I and World War II, many trawlers were used as minesweepers, the activities being similar, and both the crew and the equipment aboard already suited to the task. During the Cold War, some countries often used trawlers outfitted with additional electronic gear to monitor the activities of their enemies.