Training Island (修行の島) is an island south of Kame House. It is where Master Roshi takes his students to train because his smaller island is not enough to accommodate their needs. Master Roshi has his house transported by capsule from his small isolated island to this larger one.
The island is very big and is populated with many people. A jungle with many animals living in it is found on the island; it is in this jungle that Master Roshi throws a rock for Goku and Krillin to find.
Goku and Krillin spend a lot of time on it, before the 21st World Martial Arts Tournament, doing "training" like delivering milk across the whole island, moving dirt, swimming, dodging bees, and school work, which actually helps to fortify their muscles and minds at the same time. Krillin returned there with Yamcha, doing training before the 22nd World Martial Arts Tournament.
During the milk delivery training, Goku and Krillin are shown carrying milk in several different climates, such as desert, arctic, and others.
In Dragon Ball: Advanced Adventure, the Training Island (named Island of Training) is featured as a side scrolling stage, reenacting the competition between Goku and Krillin to find the rock with Roshi's signature on it.
In Dragon Ball Z: Attack of the Saiyans, it is featured as the first area with monsters, and is shown to be a cave. They are all very weak monsters, however at the time of going there, they are seemingly strong. As a graduation test, Master Roshi tells Krillin, Yamcha, and Tien Shinhan to find his Crystal Ball (a gift from his sister). They get there, only to be confronted by Jackie Chun (Roshi in disguise). After defeating him, they take the ball back to Roshi's house, who gives it to his students. Enemies encountered here are Giant Snake, Raider, RR Deserter, RR Ex-Sergeant, and Lantern Ghost.