Cát Bà is the largest of the islands fringing the western edge of Halong Bay and the most popular base for travellers who want to be on the water without staying in Halong City. It is a forty-minute hydrofoil from Haiphong's Bến Bính harbour, or a longer combined bus-and-ferry route via Hai Phong's outer ports.
The island has a national park covering more than half its area, with hiking trails into limestone hills inhabited by the rare Cát Bà golden-headed langur (fewer than seventy left in the wild). The southern town has hotels, dive operators, and tour boats running into the surrounding bay. The beaches just east of town are tourist-strip but the inland trails and the boat trips into the karst islands are excellent.