Dolphins are part of the toothed whales. Generally, they are among the smaller whales. Most live in salt water oceans, but some live in rivers – there are oceanic dolphins and river dolphins. Dolphins are from 1.5 metres (4.9 ft) to 4 metres (13 ft) long, but the largest dolphin, the killer whale (or orca), can be up to 8 metres (26 ft) long.