It rises at about 900 metres (3,000 ft) above sea level on the northern slopes of the Brocken.

The Ilse then rushes through the narrow Ilse valley, hemmed in to the east by the rugged, cross-topped Ilsestein, passes by Ilsenburg into the Harz Foreland, flows through Veckenstedt, Wasserleben, Osterwieck and Hornburg and discharges into the Oker near Börßum together with the Schiffgraben from the Großes Bruch.

Local folklore personifies the river as the beautiful Princess Ilse, who has her home in the rocks of the Ilsestein.

The Ilse is mentioned in literary works such as the Die Harzreise by Heinrich Heine.

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