Bukit Lawang, on the park's eastern edge, is where most people go. Sumatran orangutans are seen on guided treks from a few hours to several days, along with Thomas's leaf monkeys, white-handed gibbons calling at dawn and long-tailed macaques. Some individuals here descend from a former rehabilitation programme and are semi-habituated, which makes sightings reliable but also means guides must enforce distance — disease transmission from humans is a real risk to them.

Ketambe, in the Alas valley, is the research station area and the choice for people who want genuinely wild orangutans and a deeper forest experience. Sightings take more work; the forest is better.

Tangkahan is the elephant destination, where former logging elephants and their mahouts run patrols and visitors join river walks — a very different, more managed experience, and one worth researching for its ethics before booking.

The rivers throughout add otters, kingfishers and the tubing descent back to Bukit Lawang that ends most treks.

Sumatran tiger and rhino live here but are effectively never seen; treat any operator promising them as untrustworthy.

The drier months from May to September and December to March make trekking easier, though rain is possible year-round.

Practicalities

Access is from Medan, three to four hours by road to Bukit Lawang. Treks are booked with local guides and permits are handled through them. Choose a guide who keeps a proper distance from the apes and does not feed them — the difference between operators here is significant, and habituated orangutans have been injured by bad practice.