Sairee Beach is Koh Tao's longest and most developed stretch of sand, running for about two kilometres along the northwest coast. The beach itself is good — white sand, clear water — but the real action is the strip of bars, restaurants, dive shops, and accommodation that lines the road parallel to it. This is where most of the island's nightlife happens, where the majority of dive operators are based, and where you will find the widest range of food.
It is lively but not oppressively touristy. The dive shop culture keeps things relatively purposeful — most people here are on courses or recovering from them — and the party atmosphere is more good-natured than debauched. The southern end of Sairee, near the village, is busier; the northern end gets progressively quieter.