A cog was a type of ship that was used during the Middle Ages, mostly for trade and transport but also in war. It first appeared in the 10th century, and was widely used from around the 12th century on. Cogs were clinker-built, generally of oak. Cogs were fitted with a single mast and a single square sail. They were used primarily for trade in north-west medieval Europe, especially by the Hanseatic League. Typical seagoing cogs were from 15 to 25 meters long, 5 to 8 meters wide, and were of 30–200 tons burthen. Cogs were rarely as large as 300 tons although a few were considerably larger, over 1,000 tons.

Medieval ships were the vessels used in Europe during the Middle Ages. Like ships from antiquity, they were moved by sails, oars, or a combination of the two. There was a large variety, mostly based on much older, conservative designs. Although wider and more frequent communications within Europe meant exposure to a variety of improvements, experimental failures were costly and rarely attempted. Ships in the north were influenced by Viking vessels, while those in the south by classical or Roman vessels. However, there was technological change. The different traditions used different construction methods; clinker in the north, carvel in the south. By the end of the period, carvel construction would come to dominate the building of large ships. The period would also see a shift from the steering oar or side rudder to the stern rudder and the development from single-masted to multi-masted ships. As the area is connected by water, people in the Mediterranean built different kinds of ships to accommodate different sea levels and climates. Within the Mediterranean area during the Medieval times ships were used for a multitude of reasons, like war, trade, and exploration.