St. Agnes joins the island of Gugh by a tombolo, a kind of sandbar, called the Gugh Bar, which is exposed only at low tide. The two islands together have the smallest population of the Scilly archipelago, with 73 residents recorded in the 2001 census, and a landmass of 366 acres - 150 hectares.
The island's most notable landmark is its lighthouse, which has been converted into living accommodation and no longer contains a light. It was built in 1680 by Trinity House and was coal fired until 1790, when it was converted to oil fired, with copper lamps and 21 revolving reflectors. A plaque records original construction by Captains Hugh Hill and Simon Bayly, builders of the 1676 Lowestoft lighthouse.
The St. Agnes lighthouse was the second to be built in Cornwall (after the Lizard lighthouse of 1619). It stands 74' above the ground, and 138' above mean high water. It was superseded in 1910 by the Peninnis Lighthouse, St. Mary's. It now serves simply as a daymark for shipping.
Other landmarks include a standing stone known as the Nag's Head (probably a natural formation) and the so-called "Troytown Maze" a pebble maze thought to be of medieval date.
In earlier times many men from St Agnes earned a living as pilots, guiding transatlantic liners and other vessels through the English Channel. Now the mainstay of the economy is tourism, together with some bulb farming. Accommodations are limited, and St Agnes is the only populated island in the Isles of Scilly which has no hotel. However, it has a few B&Bs and self-catering cottages, a campsite, and a small post office and general store. It also has a pub (the Turk's Head) and a cafe, although these are closed in the winter.
St Agnes is a favourite island of visiting birdwatchers, particularly during the Scilly season of October. Among the many vagrant birds which have been found here during the month of October are the following, which were all firsts for Britain:
Among rare vagrants recorded at other times of year are the following: