Kythnos is about 100 square kilometers in area and has a coastline of about 100 kilometers. It has 72 beaches, most of which are still inaccessable by road.
The island has two significant settlements, the village of Mesaria (pop. 676), known locally as Hora, and the village of Dryopida (pop. 517). Both villages are notable for their narrow, winding, crowded roads in the characteristic Greek village style. Hora is also notable for its large Greek Orthodox Church. The island's first internet café recently opened in Dryopida. There is also a growing coastal settlement called Kanala on the east side of the island, and many of the larger beaches are settled by a handful of residents. On the north end of the island lies Loutra, a site famous for its thermal springs, which are said to have curative properties.
There is also a port town called Merichas, whose population significantly fluctuates during the year. Before the 1970s, there were no year-round residents in recent history; a Greek fisherman named Manolas Psaras and his wife Foto were the first to live in the port year-round. Today, there is a small year-round population, but during the peak of the Summer tourist season, the town becomes quite busy. Many residents of the port speak at least some English, the most popular second language. Merichas is connected to Piraeus by ferry boat, and the ride usually takes 90 minutes to four hours depending on the speed of the ship and the weather.
Kythnos was until recently considered to be one of the last Cycladic islands unaffected by the impact of tourism, but this is inexorably changing. Still, the island has not yet been overdeveloped, and in the more remote areas of the villages, traditional ways live on relatively unchanged.
Kythnos is mentioned in antiquity. Aristotle praised the island for the efficiency of its administration.
Kythnos was recently the site of a significant archaeological find. In 2003, archaeologists discovered the unplundered, untouched inner sanctum of an ancient temple, replete with over 1400 precious objects, including some made of gold, silver, bronze and terra cotta.